Okay, if you're a Dunmer, this is a lot easier. Simply aquire the robe of Drake's Pride from Senise Thindo at Tel Aruhn's living quarters. Now, with your 75% fire resistance and the new 25%, you resist whatever the flame atronach shoots, and the Dremora Lords can't attack with their weapon until they shoot you with fire.

If you're not a Dunmer, try getting the Dragonbone Cuirass at a higher level. It reflects 100% fire all its own.