To make the forge you need 30 adamantite pieces, that's about 2-4 veins of ore. [OPTIONAL] (You don't need it unless you want to make the tools or armor)
To make the pick required to mine you need 60 pieces of mythril [OPTIONAL] (You can mine adamantite later)
To make the mythril anvil you need 40 pieces of mythril
You only need to mine 40 pieces of mythril and that's you're anvil sorted! (literally five minutes!) The weapon of choice do the pirate event and get the cutlass (any good sword is decent, I just choose this as it was auto-swing and pretty fast
). Choice of armor, stick with molten but reforge your accessories to warding for an additional 20 defence.
*Fight the destroyer in a caged arena with honey, a heart lantern and a campfire for a decent buff to regen. If the destroyer is going straight up, that's your chance to attack. Don't attack if he's going across as he will likely turn around. The reason I recommend this is because it is the drones that do the most damage but their lasers do not go through walls. If the drones entering the box is too much of a problem, try standard dart traps to knock them back a bit.
You shouldn't need a forge as you can skip straight to hallowed armor doing this. Now the twins and skeletron prime would be easy as you can now craft a mega shark and hallowed armor. After defeating all three bosses make the drax, you can find adamantite to make a forge whilst looking for plantera bulbs