if you can't kill the dimensional watcher in the second room, it should spawn during stage 4 in the boss room, instead it spawn during stage 7 after a while, so the player must handle both boss and watcher... it's kinda hard since npc tank is too ****** to use aggression. also, i'm not really sure, but it looks like npc summons don't hit, they skill sometimes though.
my only piece of advice is - keep close to the door in the last room. if npcs stay there and don't go to the front, then you won't agro rb from the 2nd room. if you're lucky...