There is no set way to beat them; by that I mean you can do it with different sorts of builds. It just depends on what you are good at. Russell's suggestion to reforge is also a good idea - pumping defense is always helpful. While there are other buffs that can be more desireable depending on your skill and what you are doing, defense is never ever a detriment. Consider equipping the appropriate emblem to your offensive style, too. Can I assume you are switching Adamantine helmets to go along with your selected offensive style? If not, you should consider it.
For specific strategies, You can create a moderately sized closed box floating in the air to fight The Destroyer in. I found this method incredibly easy; just get a good melee weapon that can hit multiple sections (e.g. a flail like Sunfury or a sword that is plenty long and you can swing quickly) and swing away. I buffed for this fight but found I rarely needed to use potions. I've killed him 3 or 4 times this way. There's no need to use "Hard Mod Boss weapons" on him. Though after your first kill you can make Excalibur and then he gets really easy. Phasesabers are a pretty good weapon to use for this, and they're not hard to get.
For the Twins, I was fighting them by going areal but I never beat them. Use control gravity and feather fall potions and you can fly (though the movement can take a little getting used to), and didn't need to worry about stuff like boots/falling/obsidian shield. I just couldn't get fartherthan about 60% of the way into the fight, and then I needed to farm up some buff potions and never went back to that game. I saw several videos doing this online and it cam work. You'll want to go ranged or magic to do this, of course.
For any boss, using magic I recommend a Mana Flower (reduced mana cost and auto quaff mana pots as needed) and the crystal storm spell. For ranged, I actually prefer the Clockwork Rifle or Minishark. I do recommend some Meteor farming for a steady supply of meteor shot. Both the Clockwork Rifle and Minishark give you "free" shots, which helps preserve ammo.