Kanrethad

(This is a supplemental guide for defeating a boss in the game World of Warcraft.)

There are a lot of good Kanrethad guides and I'm not going to duplicate all that here, but I do want to point out some specific epiphanies that were helpful to me.  I downed this with Astazha on Drak'Thul (who I cannot post as for some reason) at ilvl 520 with full gems/enchants/reforge but no consumables.  Kanrethad died a bit less than 5:30 into the fight, a little before the second wave of Fel Hunters.  It took me a dozen or so tries.

I) Basic structure of the encounter:
After the Pit Lord is summoned, a demon summoning will take place every 60 seconds.  That 60 seconds just repeats over and over with the different demons.
Summon Demon 
30 seconds later Chaos Bolt
15 seconds later Cataclysm
15 seconds until next Summon Demon
Repeat.

This makes it easy to anticipate what is coming, to move your Pit Lord in advance for the Fel Hunters, or whatever.  In between those times he will put the debuffs on you every 15 seconds or so, and then do other dps stuff to you like soul fire and rain of fire.

The Cataclysms are a minute apart, and with charge on a 20 second cooldown you can easily get an additional charge in every minute when the demons come out, either for mechanics or dps on the boss.  The first slot where there is supposed to be a Cataclysm after the Pit Lord, he doesn't actually finish it.

II) I used multiple guides and spent a lot of time configuring my keybindings, macros, and weak auras to make the fight as convenient as possible, and combined that with the experience gained during wipes.  I used the methods in this video guide for dealing with the demon events specifically, as well as for my demonic circle placement.
https://www.youtube.com/watch?v=8cRZpiNBIQA

III) Use demonic circle for avoiding Chaos Bolt, not demonic gateway.  Use the placement shown in the video guide.

IV) You have a lot of heals and defensive cooldowns:
Pit Lord Heal - 220K on a 20 second cooldown.  Use it.  He'll be fine.
Healthstone.  Embertap (but sparingly, you want those Chaos Bolts.)

Twilight Ward.  Sacrificial Pact.  Unending Resolve.  Possibly Shadow Bulwark if you sacced a VW (see below, Singe Magic fans.)  Use them if things get out of hand.  Likely times are: getting aggro from a gaggle of imps, needing to eat a chaos bolt, enslave demon gets dispelled by fel hunters.  If you're encountering more than one of these in a fight before your cooldowns are up then you probably just need to practice the mechanics more.

III) Dispels.  This is an important mechanic.  It is possible to be sloppy with dispels and still get a kill, but put that out of your mind.  You can and should get this perfect.  Annoying Imp stuns are unacceptable.  Seed gets cast about every 15 seconds, and expires in 15 seconds. Fel Flame Breath is on a 10 second cooldown.  Macro it, set up Weak Auras to notify you when the debuff is on, and dispel it every time.  The agony debuff does trivial damage unless you let it tick for a very long time, and it will be dispelled whenver you cleanse the Seed, so agony can effectively be ignored.

Singe Magic is a poor substitute because it only dispels one debuff, and at random.  I sacced my imp on my kill but never used Singe Magic, and I'm of the opinion that you do not need it and could sac the VW for the additional defensive cooldown if you wanted to.

A lot of guides talk about positioning yourself to damage the boss when cleansed, but this is a distant tertiary concern, a point of polish that you do not need.  Just get the debuff cleansed. If you're out of range the Pit Lord will come over and do it, so just mash the macro and it'll be fine.  The only other thing you'll need Fel Flame Breath for is aggroing the imps and the Doom Lord, but it's on a 10 second CD and those summons are 10 second channels, so you've got plenty of notice.  I use a separate keybinds for cleanse self and damage target versions of Fel Flame Breath.

IV) Don't stand in the fire.  RoF ticks for about 3% of your life.  Kil'Jaden's Cunning helps a lot for movement without breaking your rotation too bad.

V) Demons.  I won't re-hash the whole guide.

Imps: see these as a resource to draw embers from.  Keep rain of fire on them (more than one if you need it to cover more area) and chaos bolt the boss with the embers that generates.  You're not in a hurry to get them down.  If you still have some stragglers when returning from the next Chaos Bolt line of sight, clean them up then before Cataclysm and Fel Hunters.  This is the other reason I don't take Mannaroth's Fury: I don't want more aoe dps, I want longer ember farming.

Fel Hunters:  Don't panic if you lose enslave.  You have a lot of CC.  Howl of Terror.  Shadowfury.  Banish.  Fear.  Pop an AOE CC, pop defensive cooldowns, and then use your judgment to burn or CC the fel hunters, and get your enslave back up.  If this goes south you'll need to use demonic circle to avoid Chaos Bolt before you're done resolving it all.  Just get that enslave back up in time for Cataclysm.

VI) You can soulstone in this fight.

VII) There is a repair anvil where you respawn.

VIII) Make good use of the double damage debuff on the boss following charge.  Have embers ready for Chaos Bolts, and use dps cooldowns without endangering your ability to burn the Fel Hunters (who you should also charge as shown in the guide.)

IX) Get the enslave demon glyph.  If you don't, then even with a macro you will sometimes enslave right as he is charging, and while the charge won't hit you it will still be on cooldown and you'll miss your first opportunity for double damage while interrupting his first Curse of Ultimate Doom.

X) I don't think you need the purification potion.  510 gear is easily obtainable with a couple weeks in LFR and on the Timeless Isle, and is more than enough gear to end this fight before enrage.  If you want to use pots I'd go for Jade Spirit as pre-pot and during an imp phase when you'll be farming embers with Rain of Fire.

XI) Be patient.  Master one thing at a time.  It is worth it to adjust your UI or find a new macro anytime you realize that you would benefit from better information or easier control. Don't just try and try, learn something or improve something concrete between each attempt.

Good luck!