I do use a priest with the hardiness spell, and I am using a level 17 warrior with capped gear (I am level 16 now). I also make liberal use of potions and the teleporter, and try for stealth/backstab.
Unfortunately, the bosses I am talking about still kill the warrior in about 6 hits, and kill me in abut 4. It is kind of silly, and no cimbination of tactics has proven sufficient to take it out. I even tried adventuring with another covenant's warrior leader, and he died in about 10 hits. Hurting many of these enemy bosses is proving impossible.
Guess I'll go back to playing my mage for now. At least frost spells slow and get past the armor of enemies due to the higher base damage.