Gear will help, but I promise it isn't mandatory. You just have to execute for longer, and only a minute or two more at that.
I was in Destruction spec, and sacrificed my Imp to get Singe Magic to remove the shitty Curse Kanrethad keeps casting on you.
I macro'd the Pit Lord's fel breath to my Incinerate, so that he was always casting it on CD; it makes a big damage difference throughout the fight.
Every time Kan begins casting Cataclysm, don't Charge him right away. Enjoy a 7-second break with nothing else happening and DPS the fuck out of him. When you've got like 5 seconds left, start casting a Chaos Bolt on him, and then, with it just about to cast, hit Charge, and the Chaos Bolt should land for tons of damage. Immediately begin casting a second/third if possible. You want to nuke the fuck out of him while he has the debuff from Charge; extra damage!
Every time your Pit Lord is casting Fel Breath, try to run into it briefly. You won't take TONS of damage from a tick or two (and your talents should replenish it quickly anyway). The Fel Breath cleanses you of the Magic/Curses Kan hits you with. Use SInge Magic only to remove the awful, awful curses if you manage to miss removing one of those with Fel Breath. They're seriously bad enough to wipe you if they fully tick and take effect (vacuum void zones + interrupting imp). I wouldn't gamble and 'hope' you get the third effect, Backfire, which is an increase in your damage. But shit, if it happens, nuke the fuck out of Kan.
For imps, I Rain of Fire'd the portal as they were coming out, popped Soul Shatter, then F+Brimstone, Immolate + 2 incinerates, Rain of Fire, 1 incinerate, and they were usually dead. There are some macros people use to have your Pit Lord target you and use Fel Breath, such that if you position yourself on top of the imps, the Fel Breath will wipe out the Imps in one go. I tried it once and found it messy, but it might work for you!
For Fel Hunters, ensure you have as many Embers as possible, and keep the Pit Lord the FUCK away from them, period. As Kan begins running to the portal to even begin summoning them, I sent the Pit Lord behind a pillar on one side of the room, and Demonic Gate'd myself to the other side immediately. Rain of Fire on the portal, right as the first Fel Hunter pops out, Havoc it, then pop all your damage mitigation (I macro'd Sacrificial Pact, Unending Resolve, and Shadow Ward - yes, Shadow Ward! - to one macro - super convenient) which should perfectly shield you against the Fel Hunters until they're dead. Pop that Crit Buff + any other damage cooldowns you have and nuke the shit out of the second one that comes out. At this point, Kan is following your Pit Lord to the pillar far away, and you should be Rain of Fire'ing the portal again to grab the third Fel Hunter. It's practically a wipe if you let the Hunter break your Enslave. Resume nuking the second hunter, then the third. By killing Hunters 2 and 3 with about 4 Chaos Bolts and an Immolate, Hunter 1 should be dead as well.
Around this point, Kan is usually running back to the middle for a Cataclysm. Have your Pit Lord tail him so he doesn't get LoS'd behind the pillar and fail to Charge or whatever, but ensure you take the time during the Cataclysm cast to nuke once again, and time any other Chaos Bolts you have ready to go.
If you can survive and execute the imp phase, and the fel hunter phase, you can win this encounter. You just have to do the hunter phase twice, get imps down to a science (since after the second Doom Lord, it's only imp phases till the end).
One important note - when Kan begins summoning the first Doom Lord, deliberately dismiss your Pit Lord and immediately re-enslave it. Enslave has a max timer, and it'll break later during a critical phase if you don't do it now while nothing else is happening.
For Doom Lords - EZ mode - I had my Pit Lord tank them, and periodically banished one to ease up on the damage he was taking. You can Fear one in perpetuity as well. As long as you aren't taking 9+ minutes for the fight, the Pit Lord can probably survive this for a bit, and it's far easier to manage then remembering to re-banish and re-fear while other chaotic phases are happening.
Make sure there's a Demonic Teleport ready to whisk you away for the odd Chaos Bolt.
And remember, you have a sweet, full heal ready to go on a fairly low CD sitting on that Pit Lord toolbar! He has tons of health; it's safe to use it like 4-5 times in the fight. I used it plenty during shitty phase transitions.
Remember to use a Purification Pot if you make it to 7 minutes! For me, it always happened during Fel Hunter phase 2.
If you die with Kan at 20% or less, don't release or SS! Let your Pit Lord keep doing the fight, and keep using his abilities. You still have control of him. Fel Breath on CD, Charge periodically, etc. He can win the encounter for you! I wiped with like 5-10% left and took him down with the Pit Lord.
Good luck!