I think its best to use magical harp, golden shower, greater mana potion and greater healing because you can weaken it's defense and restore your mana then use magical harp also it becomes enraged whenever it exits its biome which is bad cause it becomes faster and tries to tackle you so make a spawn point in the underground jungle and then fight it in a arena and dodge its seeds and spiky balls and chloyphlyte armuor definetly helps because its set bonus is a crystal leaf that shoots enemys automaticly and the chloryphlyte warhamer does 107 damadge so its good to use that and it is highly effected by the on fire and cursed flame effect so its also good to use cursed flames spell book and with all this I killed Plantera under two minutes

I think it's best to use magical harp, golden shower, greater mana potion and greater healing. You can weaken its defense, restore your mana and then use magical harp. Chlorophyte armour definitely helps as its set bonus is a crystal leaf that shoots enemies automatically, and it's good to use either the chlorophyte warhammer, which does 107 damage, or the chlorophyte sword, which is the best pre-Planterra sword. The cursed flames spellbook is also good, because the On Fire and Cursed Flame effects are highly effective.

Also, it becomes enraged whenever it exits its biome, which is bad cause it becomes faster and tries to tackle you. Make a spawn point in the underground jungle and then fight it in a arena and dodge its seeds and spiky balls.

With all this I killed Plantera in under two minutes.