I've been fighting this guy for around 2 weeks and this morning I did it. I thought I'd share the unique ending I had to this fight with the hopes of helping others.
Gradually I've been collecting gear and now have an Ilvl of around 509 (When I started I had 469 or something). I found this very difficult at first, I don't claim to be the best Warlock in the world.
I noticed something whilst wiping a few days ago - purely out of frustration - In some cases, you can continue to control the Pit Lord after dieing. Well this is pointless you might say - but hold on because this is what got me this quest completed.
This morning I managed to blast the bastard down to around 10% before dieing. My soulstone had gone and I was raging and once again out of frustration I started to charge him with my pit lord. I thought to myself "Holy shit. I could actually kill him..." Overwhelmed with curiosity of the outcome of this fight (As if I hadn't seen it enough over the last two weeks.) I charged and breathed him every single time I could and within minutes he was dead.
The actual fight between the Pit Lord and Kan went on for much longer than the gif lets on. He seemed to keep spawning Imps so the enslave held out.
I got the quest completed despite dieing. I'm not sure what causes the Pit Lord to be controllable only sometimes after dieing. In this case I had soul stoned beforehand but I've done it a few times without using a SS.
Other things that helped me - don't bother trying to soak Chaos bolts, just LOS it. It's so annoying to have your absorbs up and STILL be killed by it because of DOTS or ROF or some other shit.
Use macros. I won't post them here unless asked because I CBA but macro'ing the Pit Lord's heal and fire REALLY helped.
Felhunters will dispel if they face the Pit Lord. I put the Pit Lord on the right hand side of the portal and aggro'd the hunters to the left, making them run towards me with their backs to the PL.
Breasts of luck to all trying to do this. Keep at it! Gear makes it so much easier.