Close distance as fast as you can and keep him staggered until he is dead. You can do that with shield bashes or windshear or power attacks.
If you are an archer, make sure our get a good shot in before you are detected. If your sneak is weak, some invisibility potions might help.
Slow Time shout will help a lot too
If you use a shield, the elemental resistance perk will give you 50% fire resistance. You can also use a shield with resist fire, and they will stack. Resist magic also works, but doesn't add up as fast and doesn't affect an on fire effect.
Lord Stone stone and Agent of Mara will help a lot. The atronach stone would absorb the magic instead of resisting it, so that could be better depending on what else you have going on. If you are Breton, Redguard, or argonian, your daily ability will help if you use it.
Absorb health, Nightingale daily abilities
