What strategies work well against the three hardmode bosses?

Solution 1:

Personally, my loadout is as follows:

• Obsidian Shield
• Ranger Emblem
• Demon Wings
• Spectre Boots
• Philosophers Stone

I reforged my items to all have at least +3 defense each. Ranged damage might also be good. My main weapon at the time was a clockwork assault rifle equipped with either crystal or cursed bullets. The splash from crystal and the afterburn from cursed work rather well against bosses.

Preparing for battle:

• Set your spawn point to a bed in the Nurse's room, with an extremely easy exit out back into battle.
• Obtain several of every buff potion that may have even the most minor advantage in battle.
• Go to a relatively flat location outside your town, just far enough that monsters spawn, but not too far to where if you used your magic mirror that the boss would despawn.
• Wait at this location until the 'night music' starts. As soon as the music changes, you can summon bosses.
• Just before or immediately after summoning the boss, press 'B' to drink all your buff potions simultaneously.

During battle:

• If you are missing more than 100 health and you can drink a potion, do it immediately. The health potion cooldown is quite painful, and restoring yourself to full health is better than waiting for the cooldown to wear off when you're at low health.
• If any buff potions wear off, don't hesitate to press 'B' again. Potions are rather expendable, so wasting a couple potions is much better than risking dying and losing the boss battle.
• Use the gravitation potion to your advantage. It is much faster and more responsive than your wings, and does not have a limit to how high you can go. Fighting in the air is much easier than fighting on the ground, as you have 360 degrees of maneuverability.
• If you are stuck with low health and potion sickness, use your magic mirror to head back directly to the Nurse to heal up. You only have a few seconds to do this before the boss catches up to you, so be prepared to heal and immediately jump back into battle. Be sure to take the battle away from your town if the boss does catch up to you.

These are the strategies I used to defeat the bosses, hopefully they work for you as well!

Solution 2:

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.