It seems that either to low a level (15) or just not powerful enough to kill off the Boss and his Hench men. Which means that I have to repeat over and over this quest with the same results, which is insanity. I cannot leave the quest until it is completed. This sounds like catch 22.
So what happens now.. do I just quit and look for a new game that I can do? Or is their some-one who can wave a magic wand and fix this for me. 
Something similar happened in my first playthrough. If you're a rogue, use the backslash ability (the one where you teleport behind him - I forget the skill's name) as often as possible. The rest of the time, run away from him and use your stun ability (the one where you throw that glass thing. I never played much of DA2, so I don't have the ability names memorized.) Just do this evasive technique until you eventually kill him.
If you're a mage, use your ice abilities. They're your best friend. Run away when you're on cooldown and turn and face him whenever your abilities are ready.
Good luck.