For future reference, kill his brothers in the other 2 dungeons first. Then you only fight him and minions. He doesn't need nerfing, you just need to kill his brothers before doing that battle.
If you are fighting Jotun with his brothers then you are doing it wrong. You can fight Jotun alone by first fighting his brothers in the dungeons on both sides of the Hall of Valor. You do not have to do all three dungeons on the same day. You just have to make sure the people who go in the hall of valor with you has also killed the brothers.