Terraria: How to Cheese the Mechanical Bosses (Master Mode)
The Destroyer
The easiest of the three. Use the Daedalus Stormbow (drop from Hallowed Mimics) combined with Holy Arrows. Build a platform about 150 blocks in the air. The Destroyer cannot reach you, but its probes can. The Holy Arrows will rain stars down, hitting multiple segments at once.
The Twins
Focus Spazmatism (the green eye) first. Its cursed flame breath is deadly. Use a Gravitation Potion to fall up and down across the map, making it hard for them to hit you. The Megashark with Crystal Bullets is the recommended weapon.
Skeletron Prime
Destroy the Laser and Cannon arms first. Once the ranged threat is gone, the head is easy to kite. Circle around him using Wings and Spectre Boots.