I was fighting Kanrethad on my warlock trying to get green fire. after enslaving the Pit Lord i was hit by a cataclysm and died, but instead of the fight resetting my pit lord continued to fight Kanrethad as my minion. My soulstone was up and i was able to rez and continue fighting. This helped greatly with the fight.
Be warned you will have to re-summon and sacrifice your demon. also the Pit Lord will unenslave as soon as you rez.