"I'm three levels higher than him, but he kills me in just a few hits."
Maybe don't let him hit you? *grin*
Ok, ok, not possible to evade 100% of attacks, but through jumping/rolling you can avoid a lot of attacks, I think.
What potions do you have? I found when playing that I always had a few Full Resistance, +Int/Str/Dex potions, and some healing, mana, and rejuv potions. I didn't really ever need the potions against 'normal' enemies spawns (well, mostly), so I used those during boss fights. They really make tough bosses more manageable.
It's also OK to do some hit-and-run - get in near enough to hit him a few times with whatever attack you use, then jump/roll/run away, around a corner so he doesn't have line of site, pop a healing potion if you need it, fire off healing spell if you have it, etc. Just keep repeating the pattern of doing some damage, healing your own, lather-rinse-repeat.
