I beat him with with a singleton slith mage using no items on torment...not that it's easy, so much as that I have no life.
step 1: go onto his ledge and do 2 minor summons before he sees you. Buff them up as much as you can. You won't need much magic left for yourself.
step 2: attack him, or wait till your summons do. Try to keep your monsters alive with healing till he splits into three people. If one of your summons dies after he splits in half, but before he splits into thirds, run away and heal (a long trip back down). Return and distract him with monsters till he splits into thirds. Run away and heal.
step 3: There's an open space before his ledge that none of hims are usually at. Summon monsters and buff them. sneak around corner and lure only one Lystaak into the alcove with your monsters by using a long range attack on lystaak when only one Lystaak is in view. Run, quick like a bunny back to your monsters.
step 4: keep your monsters alive. They can't take much damage, but they can do lots of damage. Continue hacking at him/healing your monsters, till he's done. Run away to heal.
step 5: repeat steps 3 and 4 for the other two Lystaaks. I had lots of mage spells but minor heal was my strongest priest spell. I was heavily armored though.