What I do was first to try killing the healer(s)! So if the healer(s) are death, they can't restore HP for the others! I think ther was 1 healer, but not sure.
And after that, I go right down to Ba'al and kill him with the spell magic missile (+ passiv skill "confusion!) & also melee weapon! And in between I drink a healingpotion ...
But I was already at level 29 or 30 I think... don't remeber exactly...