Plantera

Info
Plantera is a mid-hardmode boss found within the jungle. In order to summon Plantera, the player has to kill all three Mechanical bosses (Destroyer, Skeletron Prime and The Twins). Plantera is summoned by destroying one of Plantera's bulbs that spawn in the underground jungle. The bulbs can be destroyed with any pickaxe.

Battle
After losing half of her health, she transforms into her second form, gaining damage, at the cost of her defense.

Strategy
Plantera is easy with an arena, make a big open arena that you can move around in, then fly in a circle in the middle just fast enough to bait her around.