That is the mechanics of the fight.
When you have killed all 4 rezzed Advisor's, you will enter the "MC Stage" - if you have more than 1 person in the group who is LoS of the boss, then they can/will get MC'ed. If the non-MC'ed person dies while the other is MC'ed, it *will* reset the boss and you have to do the entire encounter again starting from killing the four advisors/weapons/rezzed advisors etc.
You either have to work out a way through it, or if you have a strong "tank" type person, then what you want to do is have your tank pick up KT (interupt his Fireball cast so he'll move with you) and then have the other person go and stand behind one of the statues half way down the room and keep KT out of your LoS - if he can LoS you, he will MC one of you. If you have an old DBM you can probably duck out, dps a bit (or heal your "tank") and then duck back in. I don't have DBM and I can duck out & throw a heal or two on the "tank" person I duo with, but you always want to do it right after he says "Obey Me" and the other thing he says (I forget now).
Now, there was an amazing bug which used to MC a Death Knight and keep his/her blood worm alive (naturally, worm died, and the fight reset) - now the blood worms are no longer considered a "pet" so they can go back to solo'ing him without an issue (don't pop Army or Raise Ghoul however, they can get MC'ed).
I do not know about Hunter's at the moment, I believe it's back to *not* MC'ing the hunter and killing the pet to reset the fight.
But this *is* a mechanic - why the fight is so infuriatingly annoying, and why there are so many video's and strat's around to solo/duo him because of these mechanics.