Right, I've just this minute killed one.
I'm level 31, but bare in mind he'll be around my level too.
The way I killed him, was with the Elven Sword of Leeching that I got after killing the Stormcloak leader.
With my one handed level, it does 20 damage but regardless it absorbs 20 points of health with *each* hit.
So I would suggest using a weapon that also absorbs health, cause my health regenerated as soon as I lost it.