Great site, it brings back memories.
I have just read through the walk though. I managed to kill the Slasher of Veils before throwing the talismans into the lava in the Chamber of Virtue. I found that if I prepared a spell (missile) and travelled to level 7, my Mana would replenish. So I just kept hitting the Slasher with missiles. Not sure if it was a bug, but it worked