I believe i was lv 26/27 when i killed him.
I got my pet up and charmed the healer. I love healers <3 (did this on easy mode tough :/)
Try to hide behind pillars as much as you can, but in sight of your healer. the melee demons will come to you and you can finish them off. As for the ranged. You need to try and get closer to them, and after you killed it, run back behind a pillar again, and so on.
After they are all death (and hope your healer is alive) you can start nuke ba'al. I would count to much on ur lv10 ghost. he will get smashed
Took me about 4-5 tries to do this, with about 3 healing pots and a resist pot.