Edited and updated based on Live experiences

Tanking:One tank only. The adds are slowed and nuked by DPS.
He must have someone in melee range at all times or he will cast Magma Spit on his tank and stack Molten Tantrum on the raid. This is a quick way to wipe the raid!
When Magmaw Mangles the tank, towards the end of the Mangle timer the tank should use a defensive cooldown.

Healing: Bring an extra healer while learning the fight, because the healing demands are high during Phase 1. The key here is not overhealing in Phase 1, and regenerating mana in Phase 2.
healers need to be dedicated to healing the tank when Magmaw is Mangling him, and cooldowns may be needed.

Positioning: Magmaw is in a pool of lava and not attackable from all sides. he should be tanked facing one side of the room so that melee DPS can attack from behind, and so no one gets hit by his frontal cone breath.

Buffs: Fire resistance is useful to mitigate the large raid-wide AoE.

Key Debuffs: Parasitic Infection – any player affected will need strong healing and needs to be above 40,000 HP when it expires. They should also run out of the group.

HP: 101,352,560 (33,497,000 on 10-man)
Parasites have Parasites have 348,705 HP each on 25-man.

Enrage: None

Bloodlust: Once Magmaw is constricted by three chains and the Exposed Head of Magmaw is targetable, because at this point he will take 100% increased damage (think XT-002’s Heart).

Magmaw is a two phase fight, alternating between a survival phase and a DPS burn phase when he is constricted.

Phase 1

On the pull, the tank should stand to the left of Magmaw and the melee DPS should stand to the right, behind him. The ranged DPS should group up in one tight camp to the right-hand side of the room. Healers can stand in melee range which will minimise movement required, as long as you have enough ranged (presumably 8 as usual).

Roughly every 30s Magmaw will cast Pillar of Flame, which will deal 25,000 Fire damage to any player still inside the red swirl when the Pillar erupts. The time to run out of this is very short, so you need to be very aware of your feet. You can also watch the boss for a warning because when his cooldown is due he will rear up before throwing the fireball that causes the Pillar.

A few seconds later, a number of Lava Parasites will spawn from the site of the Pillar of Flame. These need to be slowed and killed at range, because if they reach any player they will infect that player with Parasitic Infection. This deals 17,000 Fire damage every 2 seconds and 40,000 Fire damage to anyone within 8 yards of them when it expires. What’s more, anyone with the Parasitic Infection will also spawn more Lava Parasites, and this chain reaction can quickly get out of hand.
Parasites have 348,705 HP each.

Magmaw will also cast Lava Spew, which deals 34,000 Fire damage to the entire raid per second for 3 seconds. This just needs to be healed though, and the raid should helpthe healers out by popping a healthstone or a personal cooldown to help the healers out – it just might save your life!

From time to time, the tank will be Mangled, taking masive damage every 5s for 30s. The tank needs to get dedicated healing and will need to use a cooldown as the cast ends.

After 90s, and then every 50s after that, steam will come from the floor in one segment of the room. This deals no damage but shows where Magmaw will shortly crash his head down on the ground dealing damage to anyone underneath. The melee need to move quickly to safe spots.

Once Magmaw’s head has come down, phase 2 begins.

Phase 2

Once Magmaw’s head has hit the ground, 3 melee DPS should jump onto his head (he’ll become a ‘vehicle’ at this point, so check for interface issues before assigning!). They need to use the new Constricting Chains ability they have to immobilise him, targeting it on the large Spike in the room. This has to happen at roughly the same time and be repeated, because you need three chains up and Magmaw will quickly break out of a single chain.

Once three chains are up, the Exposed Head of Magmaw will be targetable and attackable.

Damage done to the Exposed head of Magmaw is increased by 100%, so this is a short DPS burn phase. Bloodlust and personal cooldowns should be used at this point.

Once this phase is over, Phase 1 begins again. Repeat until dead. Beware because there is a threat reset as he comes out of the phase, so melee should stay away from the edge so the tank takes the first hit, and the tank must taunt as soon as possible.