Plantera

Plantera is a hardmode boss introduced in 1.2. It is summoned by destroying a pink plant that grows in the open areas of the Underground Jungle. The pink plant only spawns after at least one of the three "mechanical" hardmode bosses has been defeated. Upon death, Plantera drops a number of items, including a key that unlocks the underground jungle dungeon. When it is defeated for the first time it will always drop a Grenade Launcher with Rocket I's. Plantera changes to its second form when it reaches half health, which is 15000.

Trivia

 * "Plantera" is a parody of the American groove metal band Pantera. It was named as thus because Pantera is the developer's girlfriend's favorite band.
 * Plantera also drops "The Axe", which looks similar to the Washburn and Dean electric guitars often used by the late Pantera guitarist Dimebag Darrell.
 * Plantera's boss music also consists of heavy usage of electric guitar, the basis of metal music.
 * "Plantera" is the Swedish word for Plant (as in "to plant").
 * It is possible that the name "Plantera" is a portmanteau of "Plant" and "Terra", or a reference to the band Pantera.
 * Plantera could possibly be a reference to the giant plant from Little Shop of Horrors
 * Plantera could be a reference to a boss of the snes game Secret of Mana

Strategy
There is an easier way to kill Plantera by luring it to the surface. Before you start the encounter build a rope on the surface up in the sky. Once you lured Plantera with you just climb inside the ropes and shoot down at the boss. Plantera won't be able to chase you anymore since its tentacles require walls to hold onto. If you posses any flying capability just fly right above her at the surface while shooting down and dodging her projectiles.