99.9% of the comments in this thread do not work on legendary. Just saying.
First attempt at him I was level 85. I took an essential follower along (Dark Brotherhood initiate equipped with max-tempered gear, dragonbone weapons, all highly enchanted stuff), conjured dremora lord, AND activated dragon aspect. My char was equipped with max-tempered dual-enchanted ebony armor & weapons, am a master level illusionist, maxed-out one-handed, heavy armor, and archery. Ebony warrior handled or otherwise kept all of my helpers at bay easily.
I even hit him with a jarrin root poison to start the fight one time, it did an instant 6300 damage (minus whatever protections he has going on), it cut his health down by about 40%, but less than a minute later he had regened his health back up to nearly full.
He has either a cloak spell on constantly or else he's reflecting damage, because getting close to him for melee means you get constant damage from his "aura" (or every time you hit him).
Fus ro dah does NOTHING to the guy. Blast him off a cliff and you'll find him standing down there waiting for you at full health.
Full-of-sh** is more the sound the dragonborn's shout should make for all the comments about how "easy" this guy is to kill with the fus ro dah.
It should be a requirement in this thread to state and show proof of what level you're playing on when you "easily kicked his butt", otherwise it's a joke.