personally, I find that they're almost impossible to kill because of their annoying habit of flying away when you try to hit them. that it until I discovered a handy trick: after completing the theives guild questline, get the shadow cloak of Nocturnal. After that, reach level thirty and complete the deadric quest for Boethiah. once you have the Ebony Mail, equip it and get to where the Dragon priest is. once there, activate the shadowcloak and enter sneak mode. once invisible, get close and try to bump into it. It'll look like it knows where you are, but the idiot will just stand there and slowly die from the power of the ebony mail after attacking you only once. and that's how to beat them. However, it would be best to bring along a few potions that will weaken the power of their cloak spell. I hope some one thought that this was helpful