I geared my alt only to do this quest. I didn't kill him after trying for 2-3 hours last night. But I could get him to 2nd Doom Lord on 2 occasions. I play on EU servers from Oceania which means 400-600ms latency and it HURTS. I play demo so the leap helps with getting out of rain of fire.
What kills me:
The chaos bolt. Because I sometimes forget to abuse the portal
Dots in fel puppy phase. Because I try to kill them ASAP and forget to Singe myself.
What works for me:
Harvest life the imps. Two HoG, shadowfury, Flame Breath and spam harvest life.
Have the Pit Lord attack and tank Doom Lords. Much better than banish/fear in a hectic fight.
Use meta for the felhunters. Face them away and have the Pit Lord flame them. They never remove the enslave when I position them correctly.
I know I will have it this weekend. It's just a matter of doing everything correctly at all times. ^^