Armor and wpn doesn't matter as a Mage when killing plantera. The main thing is the arena. If going solo and gear not the best use this arena build. So build a decent open arena box arena. But not to big. U want maybe twice the size of boss. Maybe 150 x 150 block. In the middle or about 20 block high from ground. Make a tunnel 4 wide. I believe the your guy is 3 block so build one block higher. Tunnel should be just long engough to not view arena. So maybe 75 to 100 if possable.then do the same thing on other end. U want to stick a teleport on both ends with 1 or 2 block infrount of tp. Then add honey on the tp. Fire pit and heart lantern both ends, then u can add any heart mana stones near by if needed. U can use either switch or pressure plates for tp. I recommend the pressure plates. Woot arena build. Just stand on tp once u awake boss. So the plan here is just to tp back n forth using any spells down the hallway until dead. Easy peasy