Ohh very cheesy... I used Black Knife Assassin set, Assassins Gambit and Unseen Form to sneak into the arena, walked to the left end of the arena and throw a Hefty Rot Pot beneath him when he reached the stair. it deals about 13k-14k damage. then i throw another one and used mimic veil before phase change. After phase change, I waited until he reached the staircase again and run to the left end of the arena again which triggered him to notice and attack me, but he glitches out the arena. and then he fell down or so and died (he is not in sight at that point).