Boss arenas: what to build and place

The layout

Build long rows of platforms stacked above each other, with enough space between rows to jump from one to the next. Platforms let you drop down on command: Press Down to fall through. Make the arena wider than your screen, so a fast boss cannot trap you at an edge.

Clear the space around it, so you do not hit a wall while you dodge. You call Plantera by breaking a Plantera's Bulb, and the bulb grows only on underground Jungle grass once all three mechanical bosses are down. Build that arena in the underground Jungle.

Placed items that help

Place these along the arena. Each works while you are near it, so spread them out or put them in the middle.

Campfire: Life regen is increased when near a campfire. Heart Lantern: Increases life regeneration when placed nearby. Star in a Bottle: Increases mana regeneration when placed nearby. Bast Statue: Increases defense by 5 when placed nearby.

A small pool of honey gives the Honey buff (Life regeneration is increased). Put it in the middle of the arena, so you can dip in between attacks.

Class stations

Use your class station before the fight: the Sharpening Station (Usable when placed. Melee weapons have armor penetration), the Ammo Box (Usable when placed. 20% chance to save ammo), the Crystal Ball (Usable when placed. Magic powers are increased), or the Bewitching Table and War Table for summoners.

Fewer enemies during the fight

Stray enemies can ruin a boss fight. A Peace Candle gives the Peace Candle buff (Decreased monster spawn rate), and a Calming Potion does the same: Decreases enemy spawn rate.