I am about to finish the game. I am in the Hell fighting Irenicus. All summoned monsters of his died. He is "near death", but the death never comes. I did 1000 minimum points of damage to him after he became "near death". This is a bug I guess. There should be a cut scene or something?
I think you missed the trigger that ends the fight. I had that ones (in hell) I reloaded and had to try again. If you stop attacking when Irenicus turns back to himself it should not do that.
I had that too when he was not even in Slayer form. Nothing to be done about it but reload, I found. Though it WAS convenient that he just stood there and took the damage, not even bothering to fight back