Fortunately, Magmaw only has two "phases", of which only one is difficult (but on the other hand that one is really difficult). Those are a "handle the shit"-phase and a "nuke the boss"-phase.
Our raid composition was;
- Blood Dk
- Restoration Druid
- Holy Priest (me)
- Restoration Shaman
- Balance Druid *2
- Arcane Mage
- Frost Mage
- Feral Cat Druid
- Affliction Warlock
Our average item level was about ~347 (I had 343).
The thing that needs to be handled in the Magmaw fight are the Parasitic Infections (aka Larvas). Once you've got those under control, most of the rest of the fight is fairly simple and straightforward. Because of this you'll want to pick some dpsers who can help out with slowing and kiting the Larvas. We had boomkins and a frost mage, but other options include elemental shamans and hunters (slowing traps) and basically any dpser who can do the above. Because of this, this is a fight that heavily favors casters. You don't want melee to attack the larvas, so bring as many casters as you need to be able to nuke the larvas down within a 30 second period (which is the time you get before more larvas spawn).
Some tactics will tell you to have a tank for the larvas. This is only recommended if you don't have dps who can slow and kite. Having the dps handle the larvas themselves worked alot better for us, since any player who gets into contact with the larvas, even a tank, will complicate stuff (explanation given below).
Phase 1 - Handle it
This phase is about handling the larvas and intense incoming aoe damage. Get this right a couple of times and you've got this sucker.
- Lava Spew - An unavoidable aoe attack to every player in the room, deals about 100.000 damage to everyone over 3 seconds. As a healer this is the trickiest part of the fight, no doubt. This is tricky because you have to be able to together dish out 100.000 healing over 3 seconds, and most importantly, not oom while doing it. It will seem impossible the first couple of times, but you will find a way. When we finally managed to kill the sucker I never oomed, and didn't even go below 50% mana (using mana cooldowns of course). You will have to learn just how much aoe healing to push out, and try to limit it to that. Dpsers with aoe heals can also help out when needed of course.
- Pillar of Flame - Every 30 seconds Magmaw will spew fire on one raid member. It will spawn a pool of lava under that persons feet, and after a few seconds (4?) errupt into a pillar of flame. This can be on either melee or ranged, so everyone has to be ready to move from it. The Larvas will spawn from this pillar.
- Larvas (Parasitic Infection) - Anyone who gets bitten (attacked) by a Larva will eventually spawn another larva. This means that if you don't handle the larvas in a ranged manner, you'll soon be overwhelmed by them. Once bitten, you can't get rid of the infection, you will spawn a new larva throughout the fight. This is the singlemost important thing to deal with properly during the fight. Getting too many larvas will quickly make everything else in the fight impossible to deal with.
- Ignition - Ignites half the room, have everyone move out of it.
- Random Spit - A skill not mentioned on wowpedia is a random Lava Spit he does every 10 seconds or so throughout the fight (not when stunned), dealing about 30.000 damage to the target. I'm not entirely sure, but I think it hits 3 targets, and I know the damage is individual, aka stacking or not stacking doesn't change it.
Magmaw also has a couple of attacks that make him deal massive damage when no one is within melee range. This is easily handled by always have the tank tanking :P
The best way to deal with the above mechanics is to have all ranged stack up on eachother. This makes it easier to handle with the aoe damage, and alot easier to predict where the pillar will land. As soon as a pillar has spawned, have everyone move some 30-40 yards away, ranged dps dealing with the Larvas while healers try to heal up the massive aoe damage. When a new pillar spawns, move back to the original location, rinse and repeat.
UPDATE: There is also another way to deal with phase 1, which some feel is easier. Instead of having all the ranged in one group, it suffices to have one ranged outside (10 man) and the rest stacked up in melee. This way you know exactly where the Lava Spew will land and also only one person will need to move from it, reducing the risk of failure. The handling of the adds is just as important, and you should always have as many ranged needed to kill them down easily.
To get into the nuke phase, you have to have two dpsers shackle Magmaws head to a spike on the ground (ouchkerbibble). After about 1,5 minute you'll be able to jump on to his back (you'll notice the regular mount up mouse over). As soon as you have people do this, Magmaw will start chewing on the tank (Mangle), dealing massive amount of damage. Assign at least one healer to deal with this! The tank won't be stunned while chewed upon, and should use any means necessary to stay alive (heal self, use cooldowns).
The riders will get a marker (green circle) which they have to aim on the spike on the ground to shackle down Magmaws head. The faster they do this the better, since you don't want to have the tank Mangled for longer than necessary. It is worth noting that you can start doing this as soon as you've mounted, eventhough Magmaw hasn't finished his emote yet.
Phase 2 - Nuke it
Once Magmaws head is down, finish off the final larvas (they should preferrably be all dead when this happens), and have everyone start nuking Magmaw. Dpsing Magmaw at any other phase isn't unecessary, but it will overall make the fight more difficult since you want to save mana and brain power to deal with the larvas during Phase 1. If you have dpsers who are really low on mana and Magmaw still isn't close to death, it is better to have them regen than nuke during this phase. Dealing with the larvas the next time is still top priority!
There will be no damage going around, so this is the perfect time for healers to just stand around, take a breath, and regen mana through any means necessary. We had one assigned healer who topped everyone off during this phase (the shaman, since he could LB up his mana during this phase), while the others just stood idle, regening. Be ready for massive incoming aoe as soon as this phase ends, as it goes right back to phase 1 again. Just after this nuke phase, he will do a melee hit on the nearest target in melee. To avoid having a melee dying from this, have them stand as far away from Magmaw as possible when nuking. Once the tank has aggro again (there might be an aggro reset here) they can stand in their regular position again.
The first phase 1 is the trickiest, since after you have shackled his head once, you can do it again faster than before. If the first phase 1 takes about 1,5 minutes, the ones after only take about 40 seconds, so if you get the first phase 1 right, it'll only become easier (as long as you don't oom).
As you can see from the healing recount, aoe healing is what you'll do during this fight. The trick is to be able to deal with the aoe healing, while moving from the Pillar of Flame, and as mentioned not oom. On the other hand that is all you have to deal with. Our Sanctuary, Healing Rain and Efflorescence did some good aoe healing, but it also made seeing the Pillar of Flame a little tricker. Overall it is worth it however. Have someone assigned to look after the tank, someone to focus on the aoe healing and one who does a little bit of both, and you'll be set. The lightwell is gold in this fight, since it can be used while moving between the two assigned areas. I stayed in Chakra: Sanctuary throughout the fight, and actually used Inner Will for the extra movement (and to ease up mana consumption on all the CoH and PoM I used). Use PoM every cooldown, use CoH basically every cooldown, tell everyone to use the lightwell and use a buttload of Prayer of Healings. Prayer of Healing is a really mana efficient spell when you've got so many to benefit from the heal, like on this fight. When moving between places you can spam some Holy Nova, which is a great aoe heal on the run (it's not on the recount because I forgot to use it :P ). Once you have figured the bare minimum you have to heal everyone, which is based on how your other healers heal as well, you will probably have found a good way to keep mana at a reasonable level and still keep people alive. People will be low on health. Most of us healers know now that that isn't an issue. You should try to keep everyone above 30% at least, so that they won't die from the next Lava Spew aoe.
With less than 1000 mastery, I want you to note how awesome mastery seems to be for us holy priests. At nearly 10% total healing done this sure makes a difference during an aoe intense fight like this (more than PoM although PoM was up every cd). Right now I prefer mastery over spirit and crit for sure.