Hi,
although my reply may be too late for "corinth", it may be helpful for others who search for this thread.
I defeated Damian with a good melee-weapon (I had a powerfull unique king's sword) and more than 100 medium health potions. I just started to hit him, ignored all the minions he summoned, and just kept drinking health potions whenever my health got low. It was an extremely tedious end-fight and I think Larian should come up with something better in the next game.
By the way, I got the impression that Damian was faster to kill, when I saved the game from within the fight, left BD, started it again, and reloaded the fight scene.
When trying to fight Damian in one go, I always reached a point when Damian's health stopped to deteriorate at all, no mater how long we did fight. With saves and stops in between, he died faster.
