Some tips to help out on the Kanrethad encounter:
Blood Horror (not crucial)
Grimoire of Sacrifice (Sac Imp in order to gain his dispel)
Do not use Glyph of Ember Tap. Any heals you use will need to be immediate rather then trickling in.
Enslave Demon (crucial)
Place Gateway off to the right of Kanrethads gate. Off the grate about 20 yards back from it. Use this to port into the alcove there in order to LoS Chaosbolts.
Place your portal a little bit closer to the alcove. Use this to port back out as soon as Kanrethads Chaosbolt has gone off in order to maximiza DPS time on the boss.
/use [@player] Fel Flame Breath
/use Demonic Siphon
/Target Pit Lord
/Script PetDismiss ()
/Target Doom Lord
Use Charge to interrupt Cataclysm or apply extra damage taken debuff to boss
Use Demonic Siphon to heal yourself.
Use Fel Flame Breath to dispel yourself and to take extra damage given buff off boss
LoS or Sacrificial pact Chaosbolts
At the beginning, before he summons Pit Lord...Nuke the living bejezzus out of him.
As soon as he summons Pit Lord, Shadowfury. Then Enslave. While DPSing boss, set pet on Assist.
You won't need to use Charge to interrupt in the first phase so use it to apply extra damage to boss. Start casting your Chaosbolt. When you see boss is casting, Charge it. Your pre-casted Chaosbolt will land and you will have time to fit in another Chaosbolt and an Incinerate or two. This extra damage will really help you beat the enrage timer.
Move out of green fire.
The first Chaosbolt you can Sacrificial Pact.
The first Cataclysm, he is fake casting. LOL. Interrupt it anyway using the above mentioned Charge technique.
Use Fel Flame Breath and Dispell on CD. Use Siphon on CD.
As he summons Imps, position yourself at the tip of your pets tail. Lay down RoF (Rain of Fire). Use a Damage mitigation CD as Imps are spawning. Let them all spawn! Fire and Brimstone Immolate. More RoF. When all Imps have spawned use Fel Flame breath macro and they will vaporize. More RoF.
Now Boss will Chaosbolt. Use your gate to travel into the alcove and LoS it. As soon as it's cast, port back out to quickly get back on the boss.
Now he will Cataclysm. Charge it.
Next he will summon a pack of Fel Hunters (3). They will attempt to dispell your Enslave Demon. Then he will attempt to eat your face. Before he summons Fel Hunters, have 3 embers ready. Use pets Fel Flame and Siphon, then send him to go stand on the other side of the room so that dogs dont get to him. If they do just reapply Enslave, but try not to let them ever face the Pit Lord.
As those dogs are spawning you need to have RoF down to get aggro right away! I also use Sac Pac and Demon Soul here (they should both be off CD by now).
Chaosbolt, Conflag, Shadowburn first dog. He needs to go down fast.
Second dog, apply Havoc then Shadowfury dogs #2 and #3 to buy time. Chaosbolt dog #3 and finish them off with Conflag and Shadowburns (yay more embers!)
Kill them fast because Kanrethads about to cast a Chaosbolt. LoS it with your gate. (this should all be happening within range of it)
Then he will Cataclysm. Charge it.
Next he will summon a Doom Lord. Banish it, or let pet tank it.
Rinse and repeat.
You can leave to repair and come back with out losing saved progress.
Dont even bother with food, flask or pots until you have practiced the mechanics and are ready for the kill shot.
Speedy recovery time after wipes is key. Buff and Summon/Sac pet while running in. Summon boss right away and set up ports while the RP part is going on to save time.
It's a challenging fight but totally doable. It's scripted with no real RNG you so can learn what to expect and when. After a wipe consider what went wrong. Was it Strategy? Execution? Positioning? OMGWTFPANIC? Then adapt and move on.
Too long, did not read? Wait 2 or 3 patches and you will out gear it.
GOOD LUCK ALL!