I killed him friday, want to share what i did:
Ilvl 498 at the time.
Make the macros for the Pit lord, use enslave demon glyph (50% cast time reduction)
Try to stay in range of your pitlord, dont forget to dispell, use the demonic gateway for chaos bolts, only use sacrifical pact in case of emergency.
You can safely use charge 2 times before you get any Cataclysm, i stopped using it after that because i didn't wanna risk it being on cd and me dying to cataclysm (or barely surviving with UR+SP+ a teleport (else you die to falling damage)
So for the adds:
Imps stand in middle of the portal, throw a RoF down on the floor, have your pitlord Breath on you till the mobs are dead (keep RoF up and chaos bolt him in the face)
Felhunter: Put your pit lord in the corner behind the portal, position yourself just in front of the portal (In range of breath for dispells/heals)
I saved DS for this and put RoF on the portal, chaos bolt + conflag 1 of the puppies, should be dead, refresh RoF, Havoc 1 of the puppies, Conflag + Incin + Incin, finish them off with Shadowburns.
Doom Lord banish his ass (/target Doom Lord / cast Banish macro)
Now by the time the doom lord is banished, you a few minutes into the fight, i chose to drop my Pit lord here, to reenslave it so it wouldn't break on a crucial moment (as i had in earlier tries, which ended badly)
Keep nuking, keep dispelling and pay attention to your health. Use your gateway for chaos bolts, but if hes realy low you might wanna use pact to just nuke him.
Gateway, keep in range of pitlord, place lord behind portal instead of other side of room. Nuke him as hard as you can.
Precast chaos bolt when your gonna charge so you can fit 2 bolts+conflag in it to relax dps requirements