Do you have the patch installed? The patch should prevent this problem, but unfortunately if it has already occurred in the save it will not undo it.
In the seemingly unlikely event you kept your saves, this problem can be fixed if you email your save to Larian, or worked around by several methods, in addition to reducing the difficulty level; one person could pause the game after the conversation and use the shield in the quest item section of the inventory to drop the damage, a trainer can fix your hit points until you get past that fight, with your creature (and maybe a summon) you could simply reload a save before that fight until Ba'al targets someone other than your main character and then take him out (or Hide in Shadows) before he can attack again.
