"Rugal may have been immune to your fire casters." Dromaeus said to Alek. But Joel has not demonstrated that ability. Every time Rugal hit him, Joel could anticipate it. He could minimise the damage done by every attack. If, however, he was to be struck in the head by an arrow or one of your projectiles, completely unexpected, while his guard was down... how do we know he could survive that? It does not seem likely, unless he is a Daedric Prince."