Spontaneite
Terrarian
Being the obsessive researcher type, I went into Hardmode with hours of reading behind me, some more useful than others. To save you the time, I'll be giving a guide for what I have found is the easiest way to get started with Hardmode, and how to kill the mechanical bosses. Please be aware that this is as simple and short as I can manage, and for detailed guides, you should do some reading on the Terraria Wiki.
First item to consider: Melee, ranged, or magic? Personally, I went for ranged, but was careful to keep a strong sword such as Night's Edge to hand for the Destroyer's probes. Early in hardmode, you won't have 500 health, and you also will probably be lacking in the armor department. Keeping a distance from the bosses is probably the most intelligent way to go at this stage.
Second item: Which boss first? It depends on your preparations. Most suggest the Destroyer, but I personally found the Twins far easier. Both have their advantages. The biggest advantage of taking on the Destroyer first is that he will drop Soul of Might, which can be used to craft the Megashark. This is a ranged weapon which immediately changed my hardmode situation from 'barely surviving' to 'doing pretty well'.
Early Hardmode:
When you defeated the Wall of Flesh, it dropped a Pwnhammer. This can destroy the crimson or demon altars you will have encountered in your world. Destroying these altars will quickly put Hardmode ores into your world. There will be three. The weakest is either Cobalt or Palladium. The intermediate is Mythril or Orichalcum. The strongest is Adamantite or Titanium. You should destroy at least three altars. Be careful of the wraiths which will spawn when you do so - be prepared to kill them. Also be aware that breaking altars will seed the spreading biomes in new locations on your map.
If you want to save yourself some time: Before smashing altars, go to the underworld with several stacks of stone. It must be plain stone. Find the lowest point you can and lay down as much stone as you can be bothered to, and try to make multiple such areas. The reason for this is that hardmode ores will spawn according to altitude, and large amounts of stone in an area as low down as the underworld almost guarantees that ore will spawn there, leading to effortless mining opportunity for you. I first discovered the possibility for this when I found that part of my stone Hellbridge had become Adamantite ore.
You will want to craft yourself some hardmode armor. Personally, I barely took damage against the Destroyer or the Twins, but Skeletron Prime was much more difficult. You will want to up your defense. Hardmode can be difficult to survive in, at first. The above tip will likely not produce all the ore you need unless you laid down a massive amount of stone. If you're having trouble finding hardmode ores, go to the underworld and dig into its roof. The lower down you are, the higher the likelihood of finding rare ore. Any spelunker potions you may have will only help.
First - cobalt/palladium found should be used to make a pickaxe, which can mine the next tier. Any mythril or orichalcum ore you then find should be used to make an anvil. Then a pickaxe. It's more or less pointless to make yourself armor and weapons from cobalt/palladium, or mythril/orichalcum, because your aim is to move to adamantite/titanium as soon as possible.
Second - your first adamantite/titanium should be used to make a forge. Without this, you will be unable to craft many essential hardmode items. Your next priority is armor, and then a bow or repeater. I find the repeater far more efficient, as it auto-shoots. This will take quite a lot of ore. If you want, you can save time by crafting armor from one of the lower tiers instead, which are easier to find. Bear in mind that this will make you more vulnerable.
Third - You must collect souls. Souls of Light and Night are essential at this stage. You will find them in the underground hallow and corruption/crimson, respectively. Again, it is a good idea to work upwards from the underworld - simply because all the lava around provides something for enemies to kill themselves in, saving you the trouble. Souls will not be destroyed by lava, and float where they were dropped. If you find yourself overwhelmed, retreat to the underworld, where enemies haven't changed - except for Mimics, which look like chests and have valuable loot. Fighting them is a good idea.
Fourth - In the process of collecting your souls, you will encounter crystal shards in the Hallow, and some useful drops in Crimson/Corruption. If your world has Crimson, you will receive the Ichor drop from some monsters. If you have the Corruption, you will find Cursed Flame. These are both extremely useful. Use them to craft lots and lots of arrows. Crystal shards are mostly useful for bullets once you have the Megashark.
Preparing to Fight the Destroyer:
I recommend wearing Adamantite/Titanium armor, with the ranged-bonus headgear. Taking the melee piece can be tempting, as it has much, much higher defense. However, when fighting the Destroyer and the Twins, proper preparation will mean you barely get hit, so heavier damage is more important.
I recommend the Adamantite repeater as your weapon, with ichor or cursed arrows. In the absence of these, hellfire arrows. In the absence of these, unholy or jester arrows. Try to make an effort to get the best arrows. You will want lots of them. I recommend 2000 arrows to be safe, as the Destroyer has the most HP of any boss in the game and takes a long time to kill with early Hardmode weapons.
I recommend having a good sword on hand, such as Night's Edge. This is crafted from Fiery Greatsword (Hellstone Bars), Blade of Grass (Jungle Spores and Stingers), Muramasa (Found in the Dungeon), and Light's Bane/Blood Butcher(Demonite/crimtane bars).
I recommend having the Ranger Amulet. This is a drop from the Wall of Flesh. I personally had to defeat it many times for this to drop, but I consider it well worth it for the bonus to ranged damage.
I recommend Frostspark or Lightning boots. Crafting these is as follows: Hermes Boots + Rocket Boots = Spectre Boots, + Anklet of the Wind + Aglet = Lightning Boots, + Ice Skates = Frostspark boots. These allow for very fast movement and limited flight. I consider these an essential part of combat against the Twins and Skeletron Prime. They are less important for the Destroyer. The locations these items can be found are as follows:
Hermes Boots: Surface chests, Gold chests.
Rocket Boots: Purchase from Goblin Tinkerer.
Aglet: Rare in surface chests. This one is the hardest to find.
Ice Skates: Ice chests, in underground ice biome.
Anklet of the Wind: Jungle chests, in underground Jungle biome.
I recommend Wings. The easiest way to get wings is from the Witch Doctor, who will sell Leaf Wings in hardmode for 1 Platinum. If you don't have the money to spare, you should kill Wyverns in the near-space portion of the map. They are fast, do high damage, and are very agile. They should be considered mini-bosses and treated accordingly. The Souls of Flight they drop can be combined with Souls of Night or Light; otherwise, if you have a Giant Harpy Feather (dropped from Harpies), you can use it with this to make Harpy Wings. I consider the Leaf Wings the best option, as they are superior to other wings obtainable at this stage of the game and much easier to get hold of.
I recommend Charm of Myths. This is made using Band of Regeneration, a common gold chest find, and Philosopher's Stone, which is a drop from Mimics. If you don't want to search for and fight Mimics, use the Band of Regeneration. You will find the boss battles more difficult without it, as it decreases recharge time for potions.
I recommend Reforging. This is a function available through the Goblin Tinkerer, and will add bonuses to weapons and accessories. For your accessories, Armored or Warding are the best Prefixes. For Melee weapons, Legendary is usually the best. For Magic weapons, Mythical is usually the best. For the Megashark, Unreal is the best, and definitely worth the cost. Reforging can be very expensive, but will make a drastic difference for you.
I recommend Potions: Ironskin, Regeneration, Archery, Healing. I recommend Pumpkin Pie/Bowl of Soup.
Once you are fully equipped, you must prepare your Arena.
Arena Preparation: Destroyer
I've read of and tried many kinds of arena for fighting the Destroyer. What I've found works best is making a low Skybridge. You should not be able to see any blocks below your bridge, and should be a good 200-300 blocks from the ground. Your Skybridge should not be high enough for wyverns or harpies to spawn. Your Skybridge does not need to be any longer than the length of your screen. Be completely sure that your Skybridge is a substantial distance from your Base, as the Destroyer will be rooting through all nearby blocks to try to get to you, and will easily slaughter any present NPCs en masse due to the size of its body. Place Campfires along your Skybridge, and Heart Lanterns as well, if you have them. If you have an Ammo Box, use this as well.
In the middle of your Skybridge, cut out two blocks and replace them with platform. You will be shooting down at the Destroyer through this hole.
Fighting the Destroyer
The Destroyer can be summoned with the Mechanical Worm, crafted from Tasty Chunks/Vertebrae, Iron/Lead bars, and Souls of Night. It can only be summoned at night, and due to the length of the battle, I recommend summoning it as soon as the Night music begins, signalling night-time.
The Destroyer will spawn out of sight. If you built your bridge high enough, you should almost never see it. Fire down your platforms at it - the sound of impact is a metallic plinking. You should see the arch of the Destroyer's body every now and then as it attempts to reach you. Every now and then, Probes will dislodge from its segments and come to attack you. These can move through blocks, and will shoot lasers at you. This is when your sword is useful. Kill the probes as quickly as you can and return to shooting the Destroyer. Be careful to avoid falling through - you will almost immediately be barraged by the Destroyer's many lasers and will lose health quickly. Your Bridge is what protects you from this, as lasers cannot travel through blocks.
Sometimes, the Destroyer can reach you. This is rare, and usually only happens once or twice during the fight. This might not happen for you, if you don't have a collection of blocks a few screens to the left as I do. In this case jump around and try to avoid being ringed. It should fall down again and the fight will return to normal.
Eventually, it should die. Claim your loot.
Preparing for the Twins
I don't recommend any change in equipment at this time except for your weapon. Buy the Minishark and Illegal Gun Parts from the Arms Dealer. Combine these with Souls of Might from the Destroyer and Shark Fins from Sharks to make the Megashark. You should make Crystal, Ichor, or Cursed bullets for it. Reforging the Megashark to Unreal is very expensive, but worth it, as this will be one of the most useful weapons in Hardmode.
Having defeated one Mechanical Boss, the Steampunker NPC should arrive. This next part is optional but highly recommended. At night, you can buy the Blend-O-Matic from her. This allows you to combine Stone and Gel into Asphalt, which is a block that greatly increases your movement speed. If you have enough gel and stone, or the means to get more (Slime statues are an excellent way to farm gel), make yourself some Asphalt, and make a Skybridge.
The Skybridge should, again, not be too high. You don't want a troupe of Wyverns crashing your party. You want this Skybridge to be longer than the previous one. I'd say a good 300-900 blocks. With a shorter Skybridge, you will probably have to turn around and manoeuvre around the Twins. If you are using Asphalt blocks and have Frostspark/Lightning boots, using campfires and heart lanterns is kind of unnecessary. However, it's a good idea when fighting Skeletron Prime, so you may as well lay them out now. Again, Ammo Box is an excellent idea.
Fighting the Twins
The Twins can be summoned using Mechanical Eye. This is crafted from Lenses, Iron/Lead Bars, and Souls of Light. As with the other Mechanical bosses, they can only be summoned at night. Timing is less important than with the Destroyer, as they have far less health. In fact, the first time I fought the Twins, using this method, I did it without taking any damage within about five minutes. I then proceeded to kill them twice more before I ran out of Mechanical Eyes.
The Twins consist of two linked giant eyes, one red, one green. They have separate health pools and will die separately, and drop separate trophies. When their health is low they will change form, much like the Eye of Cthulhu, and will become far more powerful. The second stage is where the Asphalt + Frostspark combination is especially useful. Using only one or the other, you will have a much harder time avoiding damage. It is advisable to kill the eyes separately, so as not to deal with both of them in second stage at once. Which you kill first is optional, and I don't feel it makes much difference - but again, if you are not using Asphalt + Frostspark, it does make a difference, and you should probably kill the green eye, Spazmatism, first.
As to the actual fight - the movement of the Twins is fairly predictable. They become much faster and erratic in their second stages, but as a rule, run away from them along your Skybridge while shooting backwards and you'll have little difficulty. If you're using Asphalt + Frostspark, you are fast enough to avoid both their physical attacks and their lasers and flamethrowers, and it should be fairly easy to avoid taking damage. Again, they are faster in their second stages. You may have to dodge a strike here and there if you want a flawless victory.
Warning: Asphalt + Frostspark makes you very fast. Be careful to let the Twins catch up to you as you run. I had a very irritating moment whereupon killing the first eye, the second was far away enough that it despawned and denied me its loot.
Defeat the Twins and claim your bounty. Farm them if you like. I certainly consider them easy enough for it.
Preparing for Skeletron Prime
Again, major changes to equipment are not necessary. If you have farmed the Twins, you may have enough Hallowed Bars to make Hallowed Armor. This is pretty useful for a ranged character due to its ranged set bonus of 25% Ammo Conservation. It stacks very nicely with Megashark's and Ammo Box's ammo conservation. So I'd recommend that, if you have enough Hallowed Bars.
Skeletron Prime is a lot more difficult than the Twins, or so I found. I took damage. I actually had to use potions. It wasn't a desperate struggle for survival, but I definitely had a harder time with this boss than the other two, and got pretty low on health. If you haven't reforged all your accessories to have defensive bonuses, I strongly recommend doing so. If you don't have the Charm of Myths, I strongly recommend getting it. And so on and so forth. Having a minion can also be helpful. If you have excess Hallowed bars, you can use these with Souls of Sight (from the Twins) and a Black Lens (which rarely drops from demon eyes) to make an Optic Staff. Otherwise, the Imp Staff, forged from Hellstone Bars, is your best bet.
If you have a short Skybridge, consider lengthening it. If you have a good length Skybridge, consider lengthening it. If you haven't adopted the Asphalt + Frostspark approach, really consider doing so. It'll make your life much easier.
Fighting Skeletron Prime
Skeletron Prime can be summoned using Mechanical Skull, crafted from Bones (a drop in the Dungeon), iron/lead bars, and Souls of Night and Light. As with the others, he can only be summoned at night. Unlike the others, he will not despawn when dawn comes, and instead will become nearly invincible, much like the Dungeon Guardian. It is in your best interest to defeat Skeletron Prime before dawn, unless you fancy the challenge. This course of action is likely to herald much doom for you. Therefore, summoning the boss early at night is an excellent plan.
Skeletron Prime comes equipped with several formidable limbs. Some of these want to cut you up, others want to shoot lasers at you. Running away and dodging are good ways to deal with this. Standing still and taking it is optional, but not advisable. As you are equipped for ranged, you should not try to tank bosses. It's generally a bad idea.
Unlike with the Twins, flaunting your ability to run in a straight line is not quite as effective here. You'll find that Skeletron Prime has this unfortunate habit of appearing from odd directions, forcing you into undignified aerial scrambles. This is where having wings really becomes handy. You'll probably end up changing directions several times in the course of dodging.
It's a good idea to focus fire on his limbs, because without those all he can do is headspin at you, which is much easier to dodge. Bear in mind, though, that unlike his normalmode counterpart, Skeletron Prime's defense increases during headspinning. Consider holding fire at these points to avoid wasting ammo.
After a fair amount of dodging, liberal ingestion of potions, and mashing your keys like they've probably been mashed before, you will hopefully have beaten Skeletron Prime, the last of the Mechanical Bosses. Congratulations!
TL;DR
PC kill bosses.
Afterword
Hopefully this is of use to you. Suggestions for improvement of the guide will be appreciated. Questions are welcome. Enjoy your lives.
First item to consider: Melee, ranged, or magic? Personally, I went for ranged, but was careful to keep a strong sword such as Night's Edge to hand for the Destroyer's probes. Early in hardmode, you won't have 500 health, and you also will probably be lacking in the armor department. Keeping a distance from the bosses is probably the most intelligent way to go at this stage.
Second item: Which boss first? It depends on your preparations. Most suggest the Destroyer, but I personally found the Twins far easier. Both have their advantages. The biggest advantage of taking on the Destroyer first is that he will drop Soul of Might, which can be used to craft the Megashark. This is a ranged weapon which immediately changed my hardmode situation from 'barely surviving' to 'doing pretty well'.
Early Hardmode:
When you defeated the Wall of Flesh, it dropped a Pwnhammer. This can destroy the crimson or demon altars you will have encountered in your world. Destroying these altars will quickly put Hardmode ores into your world. There will be three. The weakest is either Cobalt or Palladium. The intermediate is Mythril or Orichalcum. The strongest is Adamantite or Titanium. You should destroy at least three altars. Be careful of the wraiths which will spawn when you do so - be prepared to kill them. Also be aware that breaking altars will seed the spreading biomes in new locations on your map.
If you want to save yourself some time: Before smashing altars, go to the underworld with several stacks of stone. It must be plain stone. Find the lowest point you can and lay down as much stone as you can be bothered to, and try to make multiple such areas. The reason for this is that hardmode ores will spawn according to altitude, and large amounts of stone in an area as low down as the underworld almost guarantees that ore will spawn there, leading to effortless mining opportunity for you. I first discovered the possibility for this when I found that part of my stone Hellbridge had become Adamantite ore.
You will want to craft yourself some hardmode armor. Personally, I barely took damage against the Destroyer or the Twins, but Skeletron Prime was much more difficult. You will want to up your defense. Hardmode can be difficult to survive in, at first. The above tip will likely not produce all the ore you need unless you laid down a massive amount of stone. If you're having trouble finding hardmode ores, go to the underworld and dig into its roof. The lower down you are, the higher the likelihood of finding rare ore. Any spelunker potions you may have will only help.
First - cobalt/palladium found should be used to make a pickaxe, which can mine the next tier. Any mythril or orichalcum ore you then find should be used to make an anvil. Then a pickaxe. It's more or less pointless to make yourself armor and weapons from cobalt/palladium, or mythril/orichalcum, because your aim is to move to adamantite/titanium as soon as possible.
Second - your first adamantite/titanium should be used to make a forge. Without this, you will be unable to craft many essential hardmode items. Your next priority is armor, and then a bow or repeater. I find the repeater far more efficient, as it auto-shoots. This will take quite a lot of ore. If you want, you can save time by crafting armor from one of the lower tiers instead, which are easier to find. Bear in mind that this will make you more vulnerable.
Third - You must collect souls. Souls of Light and Night are essential at this stage. You will find them in the underground hallow and corruption/crimson, respectively. Again, it is a good idea to work upwards from the underworld - simply because all the lava around provides something for enemies to kill themselves in, saving you the trouble. Souls will not be destroyed by lava, and float where they were dropped. If you find yourself overwhelmed, retreat to the underworld, where enemies haven't changed - except for Mimics, which look like chests and have valuable loot. Fighting them is a good idea.
Fourth - In the process of collecting your souls, you will encounter crystal shards in the Hallow, and some useful drops in Crimson/Corruption. If your world has Crimson, you will receive the Ichor drop from some monsters. If you have the Corruption, you will find Cursed Flame. These are both extremely useful. Use them to craft lots and lots of arrows. Crystal shards are mostly useful for bullets once you have the Megashark.
Preparing to Fight the Destroyer:
I recommend wearing Adamantite/Titanium armor, with the ranged-bonus headgear. Taking the melee piece can be tempting, as it has much, much higher defense. However, when fighting the Destroyer and the Twins, proper preparation will mean you barely get hit, so heavier damage is more important.
I recommend the Adamantite repeater as your weapon, with ichor or cursed arrows. In the absence of these, hellfire arrows. In the absence of these, unholy or jester arrows. Try to make an effort to get the best arrows. You will want lots of them. I recommend 2000 arrows to be safe, as the Destroyer has the most HP of any boss in the game and takes a long time to kill with early Hardmode weapons.
I recommend having a good sword on hand, such as Night's Edge. This is crafted from Fiery Greatsword (Hellstone Bars), Blade of Grass (Jungle Spores and Stingers), Muramasa (Found in the Dungeon), and Light's Bane/Blood Butcher(Demonite/crimtane bars).
I recommend having the Ranger Amulet. This is a drop from the Wall of Flesh. I personally had to defeat it many times for this to drop, but I consider it well worth it for the bonus to ranged damage.
I recommend Frostspark or Lightning boots. Crafting these is as follows: Hermes Boots + Rocket Boots = Spectre Boots, + Anklet of the Wind + Aglet = Lightning Boots, + Ice Skates = Frostspark boots. These allow for very fast movement and limited flight. I consider these an essential part of combat against the Twins and Skeletron Prime. They are less important for the Destroyer. The locations these items can be found are as follows:
Hermes Boots: Surface chests, Gold chests.
Rocket Boots: Purchase from Goblin Tinkerer.
Aglet: Rare in surface chests. This one is the hardest to find.
Ice Skates: Ice chests, in underground ice biome.
Anklet of the Wind: Jungle chests, in underground Jungle biome.
I recommend Wings. The easiest way to get wings is from the Witch Doctor, who will sell Leaf Wings in hardmode for 1 Platinum. If you don't have the money to spare, you should kill Wyverns in the near-space portion of the map. They are fast, do high damage, and are very agile. They should be considered mini-bosses and treated accordingly. The Souls of Flight they drop can be combined with Souls of Night or Light; otherwise, if you have a Giant Harpy Feather (dropped from Harpies), you can use it with this to make Harpy Wings. I consider the Leaf Wings the best option, as they are superior to other wings obtainable at this stage of the game and much easier to get hold of.
I recommend Charm of Myths. This is made using Band of Regeneration, a common gold chest find, and Philosopher's Stone, which is a drop from Mimics. If you don't want to search for and fight Mimics, use the Band of Regeneration. You will find the boss battles more difficult without it, as it decreases recharge time for potions.
I recommend Reforging. This is a function available through the Goblin Tinkerer, and will add bonuses to weapons and accessories. For your accessories, Armored or Warding are the best Prefixes. For Melee weapons, Legendary is usually the best. For Magic weapons, Mythical is usually the best. For the Megashark, Unreal is the best, and definitely worth the cost. Reforging can be very expensive, but will make a drastic difference for you.
I recommend Potions: Ironskin, Regeneration, Archery, Healing. I recommend Pumpkin Pie/Bowl of Soup.
Once you are fully equipped, you must prepare your Arena.
Arena Preparation: Destroyer
I've read of and tried many kinds of arena for fighting the Destroyer. What I've found works best is making a low Skybridge. You should not be able to see any blocks below your bridge, and should be a good 200-300 blocks from the ground. Your Skybridge should not be high enough for wyverns or harpies to spawn. Your Skybridge does not need to be any longer than the length of your screen. Be completely sure that your Skybridge is a substantial distance from your Base, as the Destroyer will be rooting through all nearby blocks to try to get to you, and will easily slaughter any present NPCs en masse due to the size of its body. Place Campfires along your Skybridge, and Heart Lanterns as well, if you have them. If you have an Ammo Box, use this as well.
In the middle of your Skybridge, cut out two blocks and replace them with platform. You will be shooting down at the Destroyer through this hole.
Fighting the Destroyer
The Destroyer can be summoned with the Mechanical Worm, crafted from Tasty Chunks/Vertebrae, Iron/Lead bars, and Souls of Night. It can only be summoned at night, and due to the length of the battle, I recommend summoning it as soon as the Night music begins, signalling night-time.
The Destroyer will spawn out of sight. If you built your bridge high enough, you should almost never see it. Fire down your platforms at it - the sound of impact is a metallic plinking. You should see the arch of the Destroyer's body every now and then as it attempts to reach you. Every now and then, Probes will dislodge from its segments and come to attack you. These can move through blocks, and will shoot lasers at you. This is when your sword is useful. Kill the probes as quickly as you can and return to shooting the Destroyer. Be careful to avoid falling through - you will almost immediately be barraged by the Destroyer's many lasers and will lose health quickly. Your Bridge is what protects you from this, as lasers cannot travel through blocks.
Sometimes, the Destroyer can reach you. This is rare, and usually only happens once or twice during the fight. This might not happen for you, if you don't have a collection of blocks a few screens to the left as I do. In this case jump around and try to avoid being ringed. It should fall down again and the fight will return to normal.
Eventually, it should die. Claim your loot.
Preparing for the Twins
I don't recommend any change in equipment at this time except for your weapon. Buy the Minishark and Illegal Gun Parts from the Arms Dealer. Combine these with Souls of Might from the Destroyer and Shark Fins from Sharks to make the Megashark. You should make Crystal, Ichor, or Cursed bullets for it. Reforging the Megashark to Unreal is very expensive, but worth it, as this will be one of the most useful weapons in Hardmode.
Having defeated one Mechanical Boss, the Steampunker NPC should arrive. This next part is optional but highly recommended. At night, you can buy the Blend-O-Matic from her. This allows you to combine Stone and Gel into Asphalt, which is a block that greatly increases your movement speed. If you have enough gel and stone, or the means to get more (Slime statues are an excellent way to farm gel), make yourself some Asphalt, and make a Skybridge.
The Skybridge should, again, not be too high. You don't want a troupe of Wyverns crashing your party. You want this Skybridge to be longer than the previous one. I'd say a good 300-900 blocks. With a shorter Skybridge, you will probably have to turn around and manoeuvre around the Twins. If you are using Asphalt blocks and have Frostspark/Lightning boots, using campfires and heart lanterns is kind of unnecessary. However, it's a good idea when fighting Skeletron Prime, so you may as well lay them out now. Again, Ammo Box is an excellent idea.
Fighting the Twins
The Twins can be summoned using Mechanical Eye. This is crafted from Lenses, Iron/Lead Bars, and Souls of Light. As with the other Mechanical bosses, they can only be summoned at night. Timing is less important than with the Destroyer, as they have far less health. In fact, the first time I fought the Twins, using this method, I did it without taking any damage within about five minutes. I then proceeded to kill them twice more before I ran out of Mechanical Eyes.
The Twins consist of two linked giant eyes, one red, one green. They have separate health pools and will die separately, and drop separate trophies. When their health is low they will change form, much like the Eye of Cthulhu, and will become far more powerful. The second stage is where the Asphalt + Frostspark combination is especially useful. Using only one or the other, you will have a much harder time avoiding damage. It is advisable to kill the eyes separately, so as not to deal with both of them in second stage at once. Which you kill first is optional, and I don't feel it makes much difference - but again, if you are not using Asphalt + Frostspark, it does make a difference, and you should probably kill the green eye, Spazmatism, first.
As to the actual fight - the movement of the Twins is fairly predictable. They become much faster and erratic in their second stages, but as a rule, run away from them along your Skybridge while shooting backwards and you'll have little difficulty. If you're using Asphalt + Frostspark, you are fast enough to avoid both their physical attacks and their lasers and flamethrowers, and it should be fairly easy to avoid taking damage. Again, they are faster in their second stages. You may have to dodge a strike here and there if you want a flawless victory.
Warning: Asphalt + Frostspark makes you very fast. Be careful to let the Twins catch up to you as you run. I had a very irritating moment whereupon killing the first eye, the second was far away enough that it despawned and denied me its loot.
Defeat the Twins and claim your bounty. Farm them if you like. I certainly consider them easy enough for it.
Preparing for Skeletron Prime
Again, major changes to equipment are not necessary. If you have farmed the Twins, you may have enough Hallowed Bars to make Hallowed Armor. This is pretty useful for a ranged character due to its ranged set bonus of 25% Ammo Conservation. It stacks very nicely with Megashark's and Ammo Box's ammo conservation. So I'd recommend that, if you have enough Hallowed Bars.
Skeletron Prime is a lot more difficult than the Twins, or so I found. I took damage. I actually had to use potions. It wasn't a desperate struggle for survival, but I definitely had a harder time with this boss than the other two, and got pretty low on health. If you haven't reforged all your accessories to have defensive bonuses, I strongly recommend doing so. If you don't have the Charm of Myths, I strongly recommend getting it. And so on and so forth. Having a minion can also be helpful. If you have excess Hallowed bars, you can use these with Souls of Sight (from the Twins) and a Black Lens (which rarely drops from demon eyes) to make an Optic Staff. Otherwise, the Imp Staff, forged from Hellstone Bars, is your best bet.
If you have a short Skybridge, consider lengthening it. If you have a good length Skybridge, consider lengthening it. If you haven't adopted the Asphalt + Frostspark approach, really consider doing so. It'll make your life much easier.
Fighting Skeletron Prime
Skeletron Prime can be summoned using Mechanical Skull, crafted from Bones (a drop in the Dungeon), iron/lead bars, and Souls of Night and Light. As with the others, he can only be summoned at night. Unlike the others, he will not despawn when dawn comes, and instead will become nearly invincible, much like the Dungeon Guardian. It is in your best interest to defeat Skeletron Prime before dawn, unless you fancy the challenge. This course of action is likely to herald much doom for you. Therefore, summoning the boss early at night is an excellent plan.
Skeletron Prime comes equipped with several formidable limbs. Some of these want to cut you up, others want to shoot lasers at you. Running away and dodging are good ways to deal with this. Standing still and taking it is optional, but not advisable. As you are equipped for ranged, you should not try to tank bosses. It's generally a bad idea.
Unlike with the Twins, flaunting your ability to run in a straight line is not quite as effective here. You'll find that Skeletron Prime has this unfortunate habit of appearing from odd directions, forcing you into undignified aerial scrambles. This is where having wings really becomes handy. You'll probably end up changing directions several times in the course of dodging.
It's a good idea to focus fire on his limbs, because without those all he can do is headspin at you, which is much easier to dodge. Bear in mind, though, that unlike his normalmode counterpart, Skeletron Prime's defense increases during headspinning. Consider holding fire at these points to avoid wasting ammo.
After a fair amount of dodging, liberal ingestion of potions, and mashing your keys like they've probably been mashed before, you will hopefully have beaten Skeletron Prime, the last of the Mechanical Bosses. Congratulations!
TL;DR
PC kill bosses.
Afterword
Hopefully this is of use to you. Suggestions for improvement of the guide will be appreciated. Questions are welcome. Enjoy your lives.
