Heroic Magmaw - 10 & 25 Man

For this guide on how to fight Magmaw hc, I will assume that you already are familiar with the normal version - if you want a quick recap, you can check out my 10 man guide on the matter. Therefore I wont talk about how to deal with everything that works the same as in normal, although you can assume that everything deals more damage (like Mangle). This guide will put emphasis on the differences to normal and what you have to think about extra carefully to get this fight right.

What is new?
Both 10 man and 25 man heroic have the same game mechanics, but you might choose to deal with them slightly differently, because you have less people to spare in 10 man.

Phase 1
Animated Blazing Skeletons will spawn from additional Lava Pillars, or rather a Meteorite called Blazing Inferno, they need to be handled by a separate tank. You can see where the Blazing Inferno will land by an effect simliar to the Lava Pillars, you should avoid them in the same way. The Lava Pillars that spawn ABS also spawn a Trail of Fire that moves slowly around the room and that needs to be avoided. The ABS cast Armageddon once they reach 20%, and should therefore be nuked at that point, regardless of how you choose to deal with them before that.

Phase 2
Nefarian will hurl shadow bolts (Shadowflame Barrage) at random raid members, which increases the aoe damage taken during this fight significantly. This is still considered the easier of the two phases since you don't have to bother about adds anylonger.

General
There are two ways to handle the pillars both in normal and in heroic. You can either have ranged stand ranged and move from the pillars in a group, left and right, or you can have everyone except a minimum stand in the melee group, which means only a few people have to run to move from pillars. The fight is usually easy enough on normal that either tactic makes little difference, but on heroic there are more pillars and more things to move from which means the fight is a lot easier with the latter tactic. Videos of the fight in heroic will show some doing the first and some doing the second, but I really do recommend the second. As long as the few ranged keep their distance and the melee group stay grouped up, it really does the handling of pillars and adds a lot easier.

25 Man
What you need;

• 1 or 2 Magmaw Tanks: To switch between Mangles. You don't actually need an extra tank for when the MT is being mangled, since Magmaw doesn't attack anyone during that time. He does however still do the occasional melee swing, which can be avoided by melee stepping out a bit. It also wipes the current tanks aggro. To make things easier you can just have an offtank taunt Magmaw during Mangle to make sure any such melee swings land on them. Since you usually can spare the extra tank in 25 man I recommend using it if you can. If you do, it is preferrable to use a second Magmaw tank who can deal some decent dps while not tanking, like a feral druid.
• 1 Animated Blazing Skeleton tank: Will stand in melee and pick up all the ABS adds.
• 1 Larva Kiter (Lava Parasites): Since most of the raid will have to focus their dps on the ABS adds and Magmaw, you will have less time to spare for the regular adds, the Larvas. The best way to handle the Larvas is therefore to have someone kite them, while also slowly killing them. This is a key task in this fight and a frost dk with Howling Blast is ideal. Of course, any kiter up to the challenge can be used, but Frost Dks really have an upper hand here. If you feel like you can spare the dps, anyone can help out on killing the Larva adds when needed of course, but if the kiter does his work well, it shouldn't be needed.
• 1 Misdirecter: Having someone who MDs the ABS to the ABS tank is highly recommended on this fight. The fight is manageable without one, but it makes it alot easier.

How to do it;
Phase 1 100-30%

• Have everyone stand in melee range, except for 4 (some say 3 is enough in 25 man, we usually go with 4 just in case someone dies) people who will stand outside to make sure that the pillars land outside and not in the melee group. It is very important that you make sure to keep these groups up, the melee group should stand close to the boss and the outside group must keep their range. Getting a Lava Pillar in the melee group usually ends in a wipe unless everyone reacts very fast.
• Every 35 seconds, Nefarian will throw Blazing Inferno at the raid (which should land outside if you've done everything correctly) that will spawn an ABS and leave a trail of fire. The Trail of Fire slowly wriggles around the room, but is fairly easy to avoid. As soon as the ABS spawn they should be picked up by their designated tank, who could be aided by an MDer to do so.
• The ABS start casting Armageddon when they reach 20% hp, which means that once they reach that health, you must be ready to be able to burn them down asap. Which way you decide to deal with the adds is really up to you, most people burn them down asap as long as it's not during a "Head Down"-phase. If you have the dps for it you can kill them through splash-aoe, and burn them only when they reach 20%. It's not recommended to tank more than 2 adds at a time, but it depends on the quality of your raid of course.
• If you have two tanks for Magmaw - Whenever the first tank gets mangled, the next one taunts Magmaw off him, and tanks until he gets Mangled, rinse and repeat. 
• Larvas will spawn and work exactly as in normal. Because you usually don't have the time to focus dps both the ABS and the larvas, you will kite the larvas. Have the Larva kiter pick them up and run them around, trying not to be hit and infected of course, avoiding the Trailing Fire and not get out of reach of the healers. Like mentioned, this is a key role in the fight, and one of the trickier.
• It is recommended to have at least one healer in the outside group, to make sure the Larva Kiter always has someone in range to heal them. Preferrably someone who can deal with healing and alot of movement - druids or priests for example. Making sure the outside people is getting enough heals will be one of the things to focus on in this fight.

Phase 2 - 30-0%

• When Magmaw is closing in on 30%, make sure to clean up any adds still left, both ABS and Larvas. No new adds will spawn during the last 30%. You should also try to time his head going down with you entering the last phase, allowing you to burn his head at once and hopefully spend alot less time in the last phase.
• As soon as he enters 30%, Nefarian will start hurling shadowbolts at the raid. I haven't found any evidence to it in tooltips, but they splash damage, which mean that you want to handle them the same way you do fireballs on Halfus, and spread out. The best thing is to start spreading out ahead of the 30% so that you're not in one big group when he starts casting the shadowbolts, as that can do a lot of unecessary damage.
• The raid damage at this point is heavy, but definitely not crazy like Chimaeron Feud or Maloriak Red phase. Make sure to have everyone stick to their assignments, because it is easy to forget about point healing when raid healing seems to be tough. Assign a cooldown rotation to make sure not all healers burn their bubbles/hymns/tranquilities at once, since that will turn into unecessary overhealing.
• As soon as you get his head down and are in this phase, is usually the best time to use Bloodlust.

10 Man
What you need;

• 1 Tank for Magmaw
• 1 Tank for ABS
• 1 Kiter for Larvas

How to do it;
The big difference between 10 and 25 man is that you have less tanks to spare, because of this you usually don't have an extra tank to handle Magmaws random melee swings when the MT is being mangled. This can be dealt with by the melee group trying to step out a little from melee range and be careful on aggro.

Phase 1 - 100-30%

• On 10 man you only need 2 people outside to make sure all the pillars stay out of melee, these two people should be your kiter and preferrably a healer. If you don't have a healer who is good enough to run and heal a dpser works just as well, but you will have to keep an extra eye on the kiter to make sure he gets his heals.
• Adds are handled pretty much as in 25 man, meaning that you can decide whether you burn them down when they spawn or let them die on splash damage. They too need to be killed asap when they reach 20%, as they will cast Armageddon which deals massive damage to the whole raid.
• Larvas will be kited around the room by the kiter.

Phase 2 - 30%-0%
Just as in 25 man;

• Try to get his head down in time with him reaching 30%.
• Start spreading out slightly before he hits 30%.
• Have some plan for how healers use their cooldowns, and don't tunnel vision on raid healing.
• If you have the head down, this is a good time to use Bloodlust.

Pitfalls - things to pay extra attention to
I've mentioned a couple of these already, but it could be worth mentioning them again.

• If you use the tactic with only a few ranged (and you should), make sure those ranged always get their share of the healing love. They are easy to forget!
• The Mangled tank takes some burst damage just as in normal (but more). It's usually easy to handle as long as you're ready for it.
• Make sure to be ready to nuke the ABS if you splash them to 20%, you really don't want that Armageddon. This seems to be extra important in 25 man, where people tend to go "ah, those guys are dpsing him already, I can continue on Magmaw".
• It's really important to keep your groups intact - melee with melee and ranged with ranged. A pillar in melee is a fairly sure wipe. Don't forget to replace ranged that die (unless you can res them of course, which usually is better)! 
• If you don't have someone who can MD ABS adds to the ABS tank, he might have to move around to get into range for taunting. This does indeed make ABS handling a lot trickier, and opens up for dangerous situations (like healers getting aggro) and everyone will make sure to help the ABS tank out.

Good luck!