So, I got my Tome the day my Warlock hit 90 after many hours of farming and went through many guides, but Ackthal's was the best. Obviously just hitting 90 I was undergeared and in the last week and a half I managed to collect a lot of gear and got up to ilvl 490. Seeing the steady progression was invigorating, just like having your own personal raid encounter.
There are a few notes I think will help people that the guides don't mention, especially timing cues:
-Don't wait too long to prepot. I made the mistake of doing this while I thought I was out of combat during a really good attempt, and when I needed to dispel Curse of Doom, I couldn't.
-Right before Cataclysm he will cast Curse of Agony followed by Seed of Corruption.
-As soon as the charge stun wears off after Cataclysm, he will cast Curse of Agony, Seed of Corruption, then a Soul Fire. After these three casts he will begin summoning his next wave of demons. I used Fel Fire Breath on myself when he was halfway through casting the first debuff after Cataclysm so that when he finished casting the second one, the Fel Breathe would still have enough time to remove that debuff. When he begins casting Soul Fire, I move my Pitlord to the corner for the Fel Hunter phase, because I know he won't be casting any more debuffs. At this point I use the Pit Lord's heal.
-Don't panic. During one of my best attempts, a Fel Hunter managed to dispel Enslave Demon. I had one Fel Hunter alive. I simply banished the Fel Hunter quickly, was lucky enough that my Pit Lord didn't use Charge on me, enslaved it as Kal was casting Cataclysm and stunned him with a couple seconds remaining. With that out of the way, I killed the remaining Fel Hunter, letting it dispel my Enslave Demon during the 3rd phase because I always re-enslave my Pit Lord during that phase while Kal is channeling his summon, anyway. The point is, you can recover if you screw up, just keep calm.
-After the 2nd Fel Hunter phase, the phase cycle gets questionable. When I reached my 3rd Imp phase, he summoned another wave of imps after that, and yet another after that. I never went farther than that before I defeated him, but once you get to your 3rd Imp phase you can blow your cooldowns and start nuking him. If there's a Fel Hunter phase at some point after the three consecutive waves of imps, they would be off cooldown by then.
Noticing that he casts 3 times after Cataclysm before he starts summoning was really key in me getting the fight down, because often times I'd send my Pit Lord away to the corner too late on the Fel Hunter phase and he'd get dispelled. Another thing to watch out for is that if you feel the need to move or kite the Fel Hunters, don't accidentally move them into line of sight of your Pit Lord. Their dispel has a HUGE range. While I was 470 last week and 490 this week (I probably spent about 40 or so attempts total before I got it), now that I know the encounter I think you could do this in full heroic blues, but it would be absolutely unforgiving.
Lastly, thank you Ackthal for this guide. I hope the additions I've added will be able to help someone, but Ackthal's guide is the best thing out there for this encounter