Unlike the above answer, I prefer to do it with melee equipment. It's a lot easier if you first get at least Mythril/Orichalcum armor and a Titanium or Adamantite sword. Set up as many heart statues as you can find (within a reasonable amount of time of course) and wire them up to 1-second timers.

I've never been able to kill the Destroyer in Molten Armor. If you're good at dodging, I recommend Menacing on all accessories. If you aren't, then go for Warding.

The good thing about the Destroyer is that he's REALLY long, which means you can camp beside him while he's digging around and rack up high damage.

Accessories... You'll probably want a Cross Necklace, maybe a Star Cloak as well (you can combine these to get a Star Veil, so you can save a slot). I'd leave off the Cobalt/Obsidian Shield and go for Charm of Myths, Warrior Emblem, Wings, and Frostspark Boots, if you have them. Lightning Boots work as well. 

I say to leave off the shield because then you will be more aware of when you're being hit. And potions couldn't hurt. Lots of potions. All those defense/damage/critical strike buffs.