You are the scum of the earth.
Use the 800mm Gustav Schwerer (or Schwerer Gustav) with HEAP (High Explosive Anti Plantera) rounds. You can fire it once ever 24hrs. in game and it pops Plantera like a beast.
Seriously though, use Armor Piercing (AP) with the Landkreuzer P. 1000 Ratte
Okay crap visit this link (https://terraria.gamepedia.com/Guide:Plantera_strategies) for viable strats.
Using a megersherk/ Magnetic Accelerator Cannon 10 with ichor+Crystal bullets