If you have no backup weapons or loot to check, jump to the battlefield and buy the best physical damage weapon you can get, preferably in a class matching any weapon specialities your DK has.
You should see the crap they sell.
Anyways I finally beat him after like 20 tries. I found out that insect swarm does damage over time, and cancels out his healing, and then I throw in periodic spikes to get some extra damage in there to finish him off.
That second battle was really hard, you dont have time to mess with the demons so they are just extra damage hitting your guys, but after a few attempts with spike + swarm he finally died.
Thanks for the information on him, I tried giving my DK a cheasy sword, but he barely hit with it and when he did it did about 10-20 dmg, so he was never much help throughout that entire battle.
Thank god for the ability to get your skill points back, the experimentation with different spells is what saved me, because lightning and those standard energy balls just werent cutting it.
