I made a very big arena,with platfoms and campfires and heart lanterns,i got the chlorophyte shotbow with ichor arrows(or at least frostburn)i ran in a circle to avoid planteras attack.
In the second phase you can stand still and shoot plantera(if you move you may shoot her tentacles,or miss.plantera doesnt have a ranged attack)ranger is the easiest way(for me)to kill plantera so use frost armor becauase the ranged hallowed helmet has lower defense than frost armor,and you aplly frostburn.
Acessories:wings(frozen wings are good)lightning or frostspark boots,a shield,charm of myths,ranger emblem