After taking so much non-lethal damage, he will fall unc. When combat ends, open up the radial menu of your fighter, choose the "Coup de Grace" option under one of the attack commands, and you should chop his head off or something like that.
Hope that helps.
During combat, the staff of striking (remember, you might have nicked it from Lareth) deals normal (Lethal) damage (at no cost, but you can use some if you want to finish him more quickly).
Also, spells deal leathl damage (magic missils ? Melf acid arrows ? didnt have time to check, my rogue/fighter sneaked him bad with the above mentioned staff, so the cbt was pretty short).
I can't even find the guy! I see the map where it shows the secret passage but all my party discovers there is the steps to the tower. I tried searching with my rogue, casting search for secret doors, nada. Any idea what I'm doing wrong?