There's an alternate strategy to this fight that is worth trying if you're a lower ilvl lock or having trouble with the mechanics. For this strategy you will ABSOLUTELY need a purification potion, otherwise you'll be getting smacked with a doom tick of 50,000,000.
1. At the start, put a demonic gateway at the top of the stairs leading to the center grate. You'll be using this for a quick escape.
2. When you pull the boss, pre-pot before the fight starts for the 4000 int bonus and so you avoid the potion lockout thus making your purification potion useless. You have 20 seconds to rape his face, so use DS, doomguard, any on use trinks and so on and nuke him down.
3. Now he'll summon the pit lord. Wait for the big guy to come out and run to your portal and warp to the stairs. Run to the bottom and the PL will follow you down. Stun him with shadowfury then enslave him. Set him to passive and move him to the bottom of the stairs inside the big square. Karenthad will cast RoF on you, but his chaos bolts and cataclysm wont hit you from this location. Note that he will run down the stairs to cast ultimate doom and a couple debuffs on you but he'll run back to the center grate right after he finishes.
4. Imps spawn now and will come running down the stairs after you. Hit them with Fel Breath and use your own aoe to burn them down.
5. Fel puppies come out next, but they wont come down the stairs after you and your big friend, so ignore them for now.
6. Now the doom spawns. He'll run down one side of the stairs so LoS him with one of the statues at the base and have your pit charge him and hit him with the breath attack. Pop cd's and burn him down as best you can. Remember to use charge and breath on cd to take advantage of the extra damage.
7. A new wave of imps will come out now, so if the doom is still up then banish him and burn the imps down like you did before. While the doom is banished this is a good time to refresh your enslave demon, so dismiss the pit, stun him with shadowfury, enslave him and focus on the doom again if he's still up.
8. Second round of puppies, you'll ignore these for now just like the first.
9. Second doom comes out, you'll deal with him just like the first. Pop your cd's and burn him and be sure to banish him if you didn't kill him before the next wave of imps.
10. Third imp wave. Now you need to react fast and clear all your debuffs and pop your purification potion. This should remove the curse of ultimate doom from you which will be close to going off at this point. When the imps run down go to work on them, then unbanish the doom and finish him off.
11. Now to deal with the fel hounds. Move your pit lord to the bottom square between the stair cases and head up top. Set up a teleport behind one of the pillars so you can LoS chaos bolt and get to the stairs if need be. Now, there will be six up, so blow your dps and survival cd's and try to kill them as fast as you can. LoS his CB with the pillar and if he starts casting cataclysm get your ass down the stairs. The fels will probably dispell your enslave at this point, but just banish or fear the pit and focus on burning the puppies down.
12. Once all the dogs are dead, you're now ready to head up top with your pit and start wrecking the boss. From this point on the only adds he'll spawn are the imps so just handle them and burn him down as fast as you can. LoS or Sacrificial Pact/Twilight Ward the chaos bolts and charge him when he casts cataclysm. The imps are a great resource to build up embers,shards, or fury, so take advantage of that to help push dps on the boss. Remember to check your pits enslave timer and re-enslave if needed and be sure to make use of charge for the extra damage on the boss.
If done correctly, this strategy lets locks in the high 470's - low 480's complete the fight. It takes a long time and a bit of practice, but it's very doable once you get the hang of it.