I haven't quite figured out how to use this practically, but a week or two ago while we were wiping on Reliquary of Souls in Black Temple, I decided to run off and have an explore. Behind the pod sort of thing that the boss comes out of is a pillar sort of thing against the wall. There's a gap inbetween the pillar and the wall that I was able to wall jump up to, and eventually after the boss had killed off nearly the entire raid the boss fixated onto me repeatedly without actually hitting me at all.
A warlock was still alive, and he said he was able to do damage to the boss while I was sitting up the back of the room fixated without taking any damage.
My point is, if somehow you can keep fixates on the same person, and have everyone stay further away from the boss than this person, and have that person wall jump up to the spot I mentioned you could do all of phase 1 without worrying about fixates or his enrages at all.