I just managed to down this with 487 ilevel gear after about 40 wipes. These are the steps I took.
Talents: soul leech, shadowfury, sac pact, burning rush, grim sac (sac the imp), kjc
1. Summon a gateway from the right centre of the circle to the left front side of the circle near the gateway
2. Summon a portal behind the pillar to the left of the giant demon gate
3. Pop dark soul and go through your opener at the start (use doomguard here), I brought him down to about 93-94%
4. When the pitlord is about to appear spam your /target pit lord /cast enslave demon macro (make sure you have the enslave demon glyph)
5. Keep nuking the boss while standing near a gateway, when he starts to cast anything use your charge and then use the stun to hop through your gateway. Keeping the boss aggro on the pitlord will stop the first chaos bolt and first cataclysm. When he starts casting curse of doom charge him again (make your whenever you charge you pop 1-2 chaos bolts). Remember to keep using the /use [(at sign)name] Fel Flame Breath macro to dispel your debuffs and use singe magic if its on cd.
6. When he starts moving to summon the wild imps, command your demon to move in front of the portal, jump through the gateway to stop aggro and then rain of fire + fel flame breath with you standing right on top of the imps, you can fnb the imps down, shadowfury and/or or spam chaos bolts at the boss. After the imps go down, charge the cataclysm and then command your pit lord to move the back right, make sure you heal and dispel while hes moving. While this is happening he will start summoning the felhunters, try and leave both your conflag charges for this and use the gateway to get to the left side of the portal. Pop dark soul. When the first felhunter appears cast immolate and conflagrate followed by havoc, chaos bolt the second felhunter you should have a backlash by now, shadowfury the first two down. While you are burning/almost finished with the last felhunter he will start casting chaos bolt, as the 2nd felhunter dies command your pitlord to return to you, use your portal to los the chaos bolt while finishing off the last felhunter, resummon the demonic portal (if you los the chaos bolt and don't move your pitlord it'll be too far and enslave demon will be removed)
7. Keep nuking until he starts summoning the doom lord, use a /target Pit Lord /script PetDismiss() macro and the enslave macro to refresh enslave. Go through the gateway to remove aggro and stand between the boss and the gate. When the doom lord appears use a breath on yourself and the pit lord will aggro the doom lord.
8. Repeat the previous steps for the 2nd imp and felhunter phases, chaos bolts may come up during these phases just ember tap, pop sacrificial pact (at full hp), twilight ward and unending resolve if needed. POP A PURIFICATION POTION IT WILL REMOVE DOOM AND GIVE YOU ANOTHER 7 MINUTES.
9. Keep the 2nd doom lord banished
10. Burn through the 3rd imp phase as fast as you can, pop dark soul and burn the boss down, try to save your shadowfuries for the 100% extra damage after charge. It will be difficult if you can't dps him down by the 3rd felhunters phase.
note: black holes may randomly appear and screw you over, burning rush can help you get out of them faster, remember to keep using demonic siphon to keep you up, remember to keep dispelling or you'll get the annoying imp that stuns you periodically