I found him a LOT easier on hardcore since I was a higher level - it made a difference. Those extra rats got me to level 3 if I killed everything that moved before going after him.
Don't forget to pick up the dagger (probably better than any weapon your hero has) at the start of the fight. Also run over to the picture and click on it to open the way to a free refill of both hit points and mana. Have your DK keep his attention (presumably has better armor class at this point) until badly hurt then switch to the hero - when both are badly hurt the hit point refill will get them both up to full hit points. In hardcore mode I didn't need a single potion and I was having two mages (strength 1) melee him.
