Boss arenas: layout and buff stations
Build a wide, flat arena for each boss and fill it with items that heal and buff you.
The layout
Stack long rows of platforms, one jump apart, wider than your screen. Platforms let you drop down on command: Press Down to fall through.
For 
Plantera, build in the underground Jungle: you call it by breaking a Plantera's Bulb, which grows only on underground Jungle grass once all three mechanical bosses are down.
Placed items that help
Each works while you stand near it. 
Campfire (crafted by hand): Life regen is increased when near a campfire. 
Heart Lantern (crafted by hand): Increases life regeneration when placed nearby. 
Star in a Bottle (crafted by hand): Increases mana regeneration when placed nearby. 
Bast Statue (in 
Sandstone Chest): Increases defense by 5 when placed nearby.
A small honey pool gives the 
Honey buff (Life regeneration is increased).
Class stations
Use your class station before the fight: the 
Sharpening Station (sold by 
Merchant), the 
Ammo Box (sold by 
Arms Dealer), the 
Crystal Ball (sold by 
Wizard), or the 
Bewitching Table (sold by 
Witch Doctor) and 
War Table (dropped by 
Dark Mage).
Fewer enemies during the fight
A 
Peace Candle (crafted at 
Work Bench) or a 
Calming Potion (crafted at 
Bottle) lowers enemy spawns: Decreases enemy spawn rate.
