Before the pull
There will be a lot of raid damage. The use of AOE heal on this fight is a very good idea. And to make those AOE worth, it’s an even better idea to spread the priests in different groups among the raid. This way, each priest will be able to AOE heal his own group, boosting the overall HPS.
You can also assign a mage to each priest. There is a lot of raid damage that will knock back their casting, and so it would boost overall DPS if each priest shield his mage during the fight.
At the pull, you have to be IN line of sight of the boss. This will give you a debuff with which you’ll have infinite mana for 3min.
Since you have infinite mana, you can pre-shield the raid before the pull.
But you canNOT put renews on any raid member (even the tank) before the pull. If you do so, the boss will directly come to you and wipe the raid.
You should use fade at the very start of the fight. You shouldn’t pull agro, but better safe than sorry.
During the fight
As I said, you’ll have infinite mana: no more downranking or mana saving, ect… You should be spamming the max ranks of your most powerful spells. Those spells are: Greater Heal, Flash Heal, Renew, Shield, Prayer of Healing, Holy Nova.
The big mistake on this fight would be to spam Prayer of Healing. Because after the first one you use, your party will be full HP and they will not have lost enough HP within the next 3s (casting time of the 2nd PoH) to make it worth. So while waiting for your party members to lose a bite more HP, you have to use another spell.
The most effective thing to do would be to put renews on the raid while you’re not casting PoH. The problem is that the priests will end up overwriting each other renews if they all do so. This means only the priests with the most heal power can actually use renew. Nonetheless, if you see that there are nearly no renews on the raid, do put yours even if your heal power is shitty. But do not overwrite the other priest’s ones!
If you cannot use renews, just use flash heal or shields while not casting PoH. The R5 greater Heal also have a high HPS but no one will ever lose enough HP to make it worth.
If your party members are stacked enough, use Holy Nova instead of Prayer of Healing. But do not spam it and wait for your party members to lose enough HP to make it worth.
If you are the bomb
If, go away from the raid. While moving don’t forget to spam instant spells such as renew, shield or holy nova.
Once you are in a safe spot, you have about 10s during which your spells will have an instant cast time. Spam as many greater heal and prayer of healing as you can during this short time