W1K
Witch
Big and wide, possibly bigger than a screen (making sure the middle of the arena doesn't turn normal underground biome or Plantera is going to rage), various rows of platforms, remove all backgrounds to massively slow Plantera down as it'll have less spots where it can hook to.I found a plantera bulb just now. Trying to brain storm an arena.
That's how we did it, it's not extensive cheesing and the fight is still there.
I believe some people suggest to use a cart, but while that can be amazing for the second phase, I believe you still need to beat the first somewhere else or the projectiles are going to destroy you.