Konstantinos you used hellspikes? I read somewhere that if you use spells during the fight the game will freeze when you kill the demon and you won't get the ending movie. So I just ran in and started hitting him with my sword. My character was a mage but at this point I was so strong that I just clicked on the demon and watched, hitting F7 for health potions every once in a while. I didn't even run around to avoid all the clones and deathknights. Didn't take very long.
