I am on the median difficulty on Xbox and I can't kill mirrak he gets all the way down in health and the he won't attack me or die.

Is he devouring the souls of dragons? Or has he passed that part?

Anyway, your only choice now is to reload a save from before you entered that area of the game.

Reload before the fight, and stop using OP weapons.

This is caused by his health decreasing too quickly. Use a weaker weapon, or amp up the difficulty.