I was in much the same position, I cheesed it by blocking off either side with chairs and candles, and also the stairway on one side so the 3 different groups (Mangoth, left demons, right demons) were seperated. When he teleported i ran into the small room and killed him away from the demons, before breaking the barricade on one side(need to telekinesis, as chairs are immune to all damage for some reason) and took the demons out one by one that way.
The boss fights have really been really disappointing to me thus far, with it either being monsters tping out of nowhere or rekilling old bosses, so I feel no shame in cheesing them if i have to.
