At this point you can have solar eclipses happen so, the best you can do is wait for it and with your world being expert a Death Sickle should drop quick, then you can build a screen wide open space in the jungle and fly in circles around plantera whilst shooting the sickles into him, all the spinning will keep him in place so all of the little sickles you shoot will be constantly damaging it.
then for extra safety measures you can also place campfires and heart lanterns around, and if you've found a grindstone use that to, but you should be able to do it without potions that way, and i do mean should, i haven't tested it but it seems like it's very possible