Just finished it..

One of the two judges and Thaos become less or more immortal after Thaos lose a certain amount of health (on normal, no idea if there are more judge on higher difficulties). The fight is simply to focus on Thaos to trigger second stage -> focus on none-immortal judge -> focus on last judge, then Thaos if still alive. He wasn't in my fight because he was gibbed on the first round. I also used figure summons to keep the second judge away from the rest of the party.