What advice do you kind folks have for The Twins and The Destroyer? I've got full adamatium armor with melee helm, Clockwork, Shotgun, and as much Crystal shot as I need, plus almost anything else. I've been fighting them on the flattened half of the world, but no special arena other then that. Do they really help that much?
Destroyer first, in my opinion. Acquire Dao of Pow, adamantite sword, and adamantite glaive. Take combat drugs (ironskin, soup, thorns if you really really want to).
Then use the souls to make a Megashark, switch to the ranged helm, and shoot the flame eye first.
And if you're going to use the Megashark, get crystals and make crystal shots for your gun, as they work very well against bosses, and Spazmatism (the flame eye in above quote) is immune to cursed bullets. To make it better, farm WoF for a Ranger emblem, forge it with damage or defense enhancing properties and wear it.
To make it even easier, farm mimics for that lengthened invulnerability cross and get a ton of hellstone/hellbrick/meteorite. Spread them down in your arena and remove any fire-immunity items. You should be damaged from the blocks. This makes you invulnerable against all bosses, but get damaged constantly by the blocks until you die or heal up. Use that to your advantage, since the hardmode bosses deal way more damage than any damaging block, excluding spikes.
Alternatively, get a ton of heart statues, lay them down in your arena and link them to pressure plates. You should get a steady supply of hearts every ten seconds or so, when you press the plates.