Before the pull
Pop a NR pot. If you are alliance, put priests in soakers group (melees).
If you are horde, you probably won't be able to do so because of shamans.
The fight
Chose the good positioning at the start of the fight. The casters and healers have to be spread out in a circle around the boss nd at max range. This will prevent you from getting the poison.
Before the enrage (at 20%), you won't have a lot to heal. Save your mana!
At the enrage, the tank and melees will take tons of damages. Preshield some melees before it happens.
Use your PI on a mage at the enrage.
If you are alliance and in a melee group, spam prayer of healing at the enrage.
If you are horde, you are probably in a caster group and so the PoH won't be of any help. In that case, spam flash heal on tanks and melees. If you saved your mana before the enrage, you should have enough mana to spam flash heal during it.