here is my strategy that I used I dont have that good of gear I posted it the other day but ill post again for you guys to see it
"I used this routine for Kenrethad and it worked like a charm once I learned the fight myself and I didnt use the strategies available, the strategy I did was, first thing is get a "purification potion"(or however many you need), next thing is you need to know is that the spell "Cataclysm" can be out ran, so if you happened to lose your enslave on the pitlord, and Kenrethad starts casting cataclysm, run down the stairs located at the back of the room, also just a note that as soon as you lose the enslave it helps to just kite all the mobs around the pillar near the stairs, so Kenrethad cant chaos bolt you and the other reason is to out run possible Cataclysm's by running down the stairs near by. I lost control over my demon on both fel hunter phases and still beat him, the next thing is when to use the purification potion, I used mine at 1.5 mins left on the debuff, just make sure that no other curse/magic/disease is on you before you use the potion, I think it only takes away one of them."