So many of you know that the xbox sometimes bugs when you are trying to kill Parthannux where he is unkillable. Well i found out how do it. When you get to the throat of the world attack him with a range weapon. He then should fly up and start attacking you. Hope this helps

or just use dragonrend and melee him when he lands on the ground from the shout wall

Well i did that before and i tried to keep killing him but he went down too 0 and he didnt die.