i use sos spec for this fight and put on some mastery items, as i like to blanket raid (or at least one group) just before purple debuff appears

id say gheal, pom on cd on tank and shields (on high mastery) are fine for purple
pi and poh spam on red/black/yellow

its also good to assign groups for purple phase, so theres no cross healing(overhealing) and adding extra stacks on ppl
another tip for purple is - alive tank is usually better than dead tank - u can trigger the explosion on tank if its needed to keep him up and no cds available, just make sure the rest of raid is high enough to survive it

i think its easier to learn this fight with dk tank (one that knows how and when to heal himself) but its not any must have
we also heal this fight with paladin tanking and its still fine on disc, he gives me hop when we have double adds