I ran in to this same problem yesterday when questing but figured out away around it. Since the quest is bugged I will post how I did it. I think it works 100% but i have only done it twice so far. Both times worked.
To know how to do it you need to know the mechanics. At 70% and 30% he is pushing all NPC to the pillars. You cant do anything when this happens and its bugged you die quickly.
If you go out and the door to the room is closed leave the instance and reset the phase. Do Not, I repeat do not reset the quest, if you so you will have to start over with the dailies and have to wait one day to do it again. (Happened to a guildy i was questing with, felt so bad for him) Resetting the phase will reopen the door.
Now to work around this this is what you need to do:
Go in and get him down to 70%. All NPC's will go to pillars. Just Die. After you die you an use medical probe but dont. Wait till he kills off all the NPC at the poles. Once he kills them all he resets back to the center and all the NPC come back as if to start again. But the fight doesnt reset completely. Now use your the medical probe. Heal up and attack again. He will now be able to be DPS him past 70% all the way down to 30% where he will put everyone at the poles and kill you again. Wait till he kills everyone and resets him and the NPC's. Use medical probe again. attack again, this time he wont do that move at either 70% or 30% and you can DPS him all the way down to 0.
Takes forever but works.
I don't know if this would be an exploit or not since the quest is bugged. I like to think of it as creative approach to the problem. The dev;s can feel free to delete this if they feel it neccessary.