You have to defeat him twice, the second round is quite hard, took me about 3 tries to kill him, i was around lvl 55 i think. if you struggle, i suggest grinding a bit on the lower levels on the tower, tentacles are easy to deal with, also, the dmg you take depends on the diff you chose, Easy - Enemy dmg multi 1x, Normal 2x, Hard 5x, Ultimate 10x, you can go to the basement where you craft and in the room with the crystals when you enter to your right is a crystal where you can change your game diff. so if you play on anything else then easy, switch to easy also note, that is the last boss :]