This is the fight that our group had the most difficulty with, bar none. We spent several weeks on this jerk. Healers and tanks all need to be spot on with their timely use of healing and tanking cd's, and dps need to communicate with the healers so a crackle (big nasty raid wide aoe that hits for about 80%+ of everyone in the raids HP) doesn't go off when they're not ready for one. The fight isnt especially difficult to heal once you get used to a few things. First, make a point of pre-shielding any raid members who have either low max HP or are prone to taking extra damage right after a crackle somehow. Additionally it is important that the tanks have some sort of damage reduction for every Crackle, as it is common for a fire breath to come right before or after a crackle, which in conjunction with the crackle damage would be enough to easily kill a tank. at the very least, make sure you shield both tanks for every crackle.
Phase 2 is actually fairly easy if you're a Discipline priest, I tend to shield myself and my pillar members as we become submerged in lava, but once we've made it onto the pillar all of the healing can be done via smite/holyfire healing, though I usually have a prayer of mending bouncing around as well simply because it is such an efficient heal.
Phase 3 consists of keeping both tanks alive as best you can, I primarily heal the Nefarian tank, so I'm free to smite heal for most of the fight, dodging the spreading fire is key in this phase, I tend to shield as many raiders as I can before each crackle so long as I have the mana for it, the key in this phase though is control, dont push a crackle unless you're ready, preferably just after a fire breath, so you know that another breath won't happen for a while, giving you time to heal your tank after the crackle.
This is a pretty rough fight, it isn't any more difficult than the other two end bosses, but if you ARE going to die to him, you're likely going to do it within the first 45 seconds of the encounter. Be prepared to fly back up to his lair quite a few times in quick succession :]
Anyway, the first phase is just about keeping your raid pretty spread out, as that stupid forked lightning he casts does more damage the more people it hits. Heal everyone between each lightning burst and try your best not to get caught in the cyclones that make circles around the platform. I run holy for this fight, as I can leave my lightwell near the tank so that even if I do get blown off of the platform, some healing can be done.
If you manage to survive the first phase (until he is reduced to 80% health) then you will have a nice lull in healing. Phase 2 has a constant raid damage component, but it starts out fairly benign, simply put out controlled AOE heals as necessary and continue to dodge tornadoes and you'll be golden.
Phase 3 is a clusterf*ck, the best we could manage was to mark the 3 healers so that the rest of the raid knew where to fly if they needed to be healed. Holy is again very helpful in this phase as circle of healing can be fired on the fly and does tremendous amounts of overall healing. At this point its up to your DPS to kill the boss quickly enough for the entire arena not to be filled up with lightning clouds.
In my opinion, this big guy is the easiest of the raid end bosses. Success depends on a few factors, your raids ability to stack up when necessary so as to facilitate the interruption of the Worship ability that he causes random raiders to channel, and your ranged DPS' ability to deal with all of the adds present in this fight. This is a fight I recommend Discipline for, it allows you help the ranged DPS bring down the adds faster while keeping your add tank healed via atonement (those awesome heals you get from smiting enemies). There are a few places in which Power Word: Barrier is especially useful during the fight, you can either be safe and save it for the final phase, or use it on the first set of shadow AOE damage pulses that Cho'gall will regularly perform and then hope it is cooled down by the final phase.
The ability to smite heal is also fairly helpful during the final phase as well, as the tentacles that come out of the ground that must be killed actually have fairly low Health, so even a little priest spamming holy damage spells on it will speed up the tentacle killing process considerably. Spam casting power word shield during the final phase is also a viable tactic, it will give your other healer(s) time to top off anyone who was a little too injured.
Anyway folks, new bosses tomorrow! Our raid group is scheduled to go and try out some of the Firelands bosses this Wednesday evening, so I'll have a post up about the experience sometime tomorrow night, that's all for now!