I just did Infiltrating the Black Temple today. A couple resources I used were a Youtube video by Adam Becker and Icy Vein's Kanrethad Guide by Ackthal. Many thanks to the two!
I got it just fine, and it was an enjoyable experience. I feel compelled to say this is a post-nerf kill, seeing as how my ilevel was 537. I killed him in the second Imp phase and felt a brief moment of "That's it?" which was fully replaced with evil glee very soon afterward!
I popped some cooldowns at the start (Doomguard, Berserking, trinket proc+Chaos Bolts) and Feared the Pit Lord (glyphed) for even more DPS at start. Just as Fear wore off, I Enslaved (also glyphed) the Pit Lord and continued attacking. My macros were:
#showtooltip Enslave Demon
/target Pit Lord
/use Enslave Demon
This gets the Pit Lord in action right away.
/use Fel Flame Breath
/use Demonic Siphon
This keeps the Pit Lord's abilities on cooldown.
This interrupts Cataclysm and I put it right next to my Chaos Bolt.
When the Imps came out, I just kept Rain of Fire on them and used another macro:
/target Fel Imp
/use Fel Flame Breath
Before the Felhunters came out, I sent the Pit Lord behind the gate using /petmoveto and avoiding my Incinerate macro until all three Felhunters were out and aggroed onto me. I used Havoc>Chaos bolt to kill two and DPS'ed down the third normally, no problems. In one of my failed attempts, I lost the Pit Lord to Felhunters because I didn't aggro them.
When the Doom Lord came out, I just had the Pit Lord attack it, used my gate to drop aggro, and continued DPS on Kan.
Overall, I just stayed out of fire and kept DPS going, using Singe Magic on cooldown and the occasional Ember Tap to stay up. I had some problems with Demonic Circle not working sometimes causing me to wipe, so I just started using my gate to avoid Kan's Chaos Bolt.
Looking through Damage Taken: Excruciating Agony for 539k and Seed of Terrible Destruction for 254k, so more Singe! One Chaos Bolt hit me for 460k, and I didn't even notice it. Rain of Fire damaged me for 101k, so I'm not perfect on getting out of fire.
All in all, not bad. I liked the story a lot, too. It was very informative about the nature of Warlocks.