I played some expert mode for a bit and was extremely frustrated.
Slime King was hella easy as long as you get a stack of 99 grenades and a good flat amount of land to fight in. You practically won't get hurt from the slime king. It's the easiest expert boss there is.
Eye of Cthulhu was a pain to try defeating, I need post platinum+ranged weapon and a mix of grenades and a minimum of iron armor. You practically also need all the buffs you can manage to fight the Eye of Cthulhu.
Now as I try platinum armor against either Skeletron or Eater of Worlds... I can't kill either of them with platinum tier weapons and grenades. I try it again with buffs, jester arrows, and all warding accessories and still failed. I try fishing for the reaver shark, getting molten gear, and trying again and finally can manage. Is this what you're suppose to do here? The order seems off...
Another way is to instead of trying to fish is to make a crimson world and fight the brain of cthulhu with jester arrows. It's a lot easier than skeletron or eater of worlds. Then you can try fighting Skeletron and or Eater of Worlds with crimtane tier gear... Also a weird order.
Once in hard mode, I take my time to get the now possible Frost Armor converting my titanium or adamantite to that type of armor and using that tier repeater against Destroyer. With buffs and jester arrows, I managed, though holy arrows work just as fine too if you don't want to wait for night. Buffs are needed to make sure you'll live a little longer.
With the Megashark, the twins are easier to deal with and so is Skeletron prime. The frost armor's frostburn effect really helps out killing. The only problem I had was taking the time in order to achieve these armors by looking for altars, waiting for it to rain to find ice golems in snow biome, basically how slow paced expert mode is...
I then move onto melee chlorophyte armor for highest defense and manage to kill plantera with megashark and ichor bullets. But finding a plantera bulb is ridiculously tedious and hard and luck based... But I have noticed how in a small world, there seem to be a hella ton more bee hive biomes to fight queen bee which I've ignored.
Golem is pretty easy once you farm vampire knives in a crimson world but it's a pain on expert mode since everything hurts so much even with chlorophyte armor and not having a way to heal that effectively yet.
Now there are only a few more hard things once you get Beetle Armor. Scale Mail is wonders for melee damage base and Shell is wonders for any class not melee based. All you need is a minimum of 90 defense points and the rest can go to your offense. I find this really dumb because not everyone wants to use beetle armor.
It seems quite literally impossible to fight moon lord on your first try, the only reason you fight the pillars is cause you need the fragments for the weapons. So after getting beetle armor, just get your lunar weapons from fragments.
In order to fight duke fishron you need an arena with the best wings you can get, or no arena and acquire cosmic car keys from martian invasion for the ufo mount. The infinite flight is the best thing you can practically use against duke fishron.
Having little defense makes it practically impossible to kill duke fishron, but you must kill duke fishron before you fight the moon lord. Melee people can manage killing duke with the spear Daybreak because that DAMAGE OVER TIME stacks up to 200 per tick and is insanely awesome. Mages use nebula blaze because it's the strongest mage weapon at this point in the game before acquiring Razorblade Typhoon. Ranged characters should use Vortex beater because it's so much more simpler to use chlorophyte bullets to kill duke fishron with as he's ridiculously mobile. Now as for Summoners, they can manage with either spooky armor to be able to summon more or stay with beetle shell and tank while damaging duke with either summoning weapons. They're somewhat about the same but cell staff is better in my preference here because that damage over time and how the cells teleport in front of duke are much more reliable when duke is enraged in his 3rd stage of low hp. I would preferably use spooky armor and 2 summoning accessories (papyrus scarab and necromantic scroll) because it's better to have more damage somewhat at this fight.
You absolutely need shrimpy truffle from duke fishron to fight the moon lord. Your accessories are a little flexible but also deadlocked. Accessories required for a must... 1. Cross Necklace or Star Veil. This item is the best accessory in terraria what so ever. You can't be taking that many hits against the moon lord.
2. Anything that gives you immunity to knockback. If you get hit, chances are, you're likely going to get hit AGAIN and having your movement speed reset while trying to kite the moon lord and be sentenced to death. The best shields of choice are Paladin's Shield for a whopping 12 defense when warding. Or Ankh Shield for 8 warding and perks of immunity to most debuffs not in 1.3
3. Shiny Stone. This thing will make you a tank god. Stay still, do nothing, for hours, for minutes, and you won't die I promise you. All you need is at least a 20 block long platform above your head to shield you from the middle eye's death laser.
4. Celestial Stone / Celestial Shell. This hybrid item gives both 10% overall damage and 8 defense if reforged warding. Very simple, damage and defense, whatever you'll need. Shell making you become a wolf at night gives even more extra stats while it's night time for more +++ helping vs moon lord expert mode solo.
Edit : The hp regen also helps, so there's so much that's good about this item. You can practically use them both since they do stack indeed.
5. The last 2 accessories are up to you to decide. Worm Scarf isn't necessarily a must but it does help. Frozen turtle shell could replace worm scarf even if it's just 25% reduced damage while you're under 250 health (or 300 with life force). They can be damage accessories but make sure you at least have 90 or 100 defense!
6. Wings are completely useless. They're a waste of stats you can gain from getting anything else. The reason why you don't need them is because they don't have infinite flight. You requires Shrimpy Truffle. This is the most amazing mount there ever is for the moon lord. You can fly infinitely in the air. And once you're below 50% of your health, you're basically "flying on asphalt" like a unicorn no-clipping in air. You simply need the air cleared of floating islands so you can simply fly straight left and right when needed. And you'll barely get hurt anymore at this point. But because you're flying around forever now, you can swap out shiny stone.
7. Master Ninja Gear. I absolutely love this item, the chance to not get damaged basically acts like titanium armor's shadow dodge. This will guarantee you to dodge more than dozens of hits you would have taken from the moon lord. And greatly it stacks well with star veil and or cross necklace for invincibility frames.
The breakdown on how to fight moon lord expert mode solo. After killing all four pillars, you can make a make-shift horizontal wall over your head of at least 20 blocks long should be good enough. Make sure you're not flying your head into this wall and not too far below it. Majority of moon lord's damage is already taken care of and don't freak out. You will not die. If you're running low on health, as I said, don't panic, just wait and DO NOTHING. You'll outheal the damage he does to you and the most damaging attacks are blocked out by the horizontal wall above you. Now if you're not using solar eruption, I suggest you should have made the moon lord wall before hand and applied actuators and wires and walls and a switch to control your solid block wall above you. You should only need like 2 to 5 actuators at most. The wall only needs to be 1 block thick. The main trick first here is to try killing all 3 eyes at exactly the same time. Once you kill all three eyes at the same time, the next easy part comes, HIT AND RUN. Just fly away on your fishron mount. You'll barely get hurt. If you fall low on health, use a greater or super healing potion. But it shouldn't really be used until you're under 100 health. With the new patch, moon lord will oftenly teleport to you because you outfly him in speed with your fishron mount while you're under half of your health. You can abuse this to your advantage by aiming at his core without needing to aim too randomly. With the set up above, you can practically use any weapon as long as it deals over 500 DPS and has a base damage well over 100. You don't need much set up at all for the moon lord as of now, he's practically now easier than the mechanical bosses when fighting him this way. There is no need to use the nurse, honey, spike traps, wooden traps, lava, heart lamps, or even heart statues + heart reach potion. Shiny Stone replaces all of these items for you in 1 accessory during the fight. You can use a star in a bottle as a mage or summons if you want to increase your damage a bit. But I don't recommend using summons since they can't really control their damage. Cause once 1 eye comes out, all of them need to come out. And you have to start taking off. There's no pattern to how you need to fly literally. Just fly straight left and or right and you're good to go. Perfect fight without cheese tactics and minimal building required.
Now, I wonder who will read this and use this information correctly?
Edit : Do make sure when fighting the moon lord, you're stocked on at least 2 to 3 of each buff potion so they don't run out depending on how fast or slow your DPS is. It should take probably a minimum of 2 potions, and 3 max. Buffs needed or that help a lot are well fed, regeneration, summoning and or bewitched if you use summons, flasks (for melee), life force is a must for extra hp, and iron skin is a must to add more to your defense. And there are quite a lot of other buffs you can use for your own damage depending on your choice of weapons.