We actually put a proper night in on this yesterday and killed it with less problems than we experienced previously with our sporadic "let's give it a go lol" efforts.
Tanks: Blood death knight and Protection warrior.
Healers: Holy priest, Restoration druid and Restoration shaman.
DPS: Survival hunter, Retribution paladin, Affliction warlock, Shadow priest and Feral druid.
The key to this, we found, was getting the balance of the DPS right and (unfortunately) this is going to be the same for every single raid because I can't possibly tell how good your damage dealers are. However, should you spend a few wipes getting to grips with that then you'll find the fight a hell of a lot smoother.
Here's the set up:
With the exception of the hunter and the resto druid, everyone is in melee range. The DK tanks Magmaw throughout, with no switching, while the warrior tanks the constructs. The hunter deals purely with the parasites and the druid heals both her and himself while targeting the melee group with Efflorescence. The Holy priest concentrates on the Magmaw tank, helped by the shammy who concentrates on the Protection warrior and the raid. Having everyone grouped up helps with this significantly. Any mix of healers/tanks should be okay, but I'd personally use a paladin or DK on Magmaw (self healing when being eaten) and try to avoid using a Holy paladin.
So long as everyone is tight on the edge of Magmaw's fissure, pillars will only ever target the hunter or the druid. The exception to this is that it will occasionally target the warrior when he comes out to pick up the construct. This is a nuisance, but can be mitigated by good timing with pillars and our hunter is also very good and misdirected pretty much all of them. All told, though, I cannot stress this enough; keeping tight to the edge of the fissure is the key to avoiding pillars in the melee. Alas, on previous nights we've found that our 2nd raiding hunter has been a bit of a liability due to her dead zone and the problem almost disappeared in her absence.
The key for us was the right allocation of DPS to the constructs so that they went down in a timely fashion. In our case, we designated the Shadow priest, warlock and paladin to DPS them down and found that this kept them very much in line. The Feral targeted Magmaw throughout, until he neared 30% and we wanted to get a head phase burn. Essentially, we were seeing constructs drop to around 30% at worst before another spawned, and the construct tank should be clear on what he wants targeted for the head phase. For the most part, you want everyone on the head when it's down and you should only ever be otherwise if you already have two constructs up when it starts.
So long as you're keeping up with the healing, Magmaw will be below half health relatively quickly. You want to push him to in and around 35% and wait for the next head phase for the final burn with all personal cooldowns blown, including Bloodlust/Heroism/Ancient Hysteria. At this point, everyone spreads out and continues to burn away the last of his health. There will still be parasite spawns, but the Protection warrior now picks these up and uses cooldowns to survive the damage. It's likely you'll have a construct up (we had two) and these should also be ignored. If you have a raid wall (Divine Guardian and Spirit Link Totem is best), activate it and finish him off.
We got excited and forgot the raid wall, got surprised with the parasite spawn and suffered the usual excitement with a final phase burn and near-kill... Yet still finished him off with a near-death Heroic Leap which saw an undead carcass die in mid air for great justice. As a quick note, I should also say that we saw none of these bizarre swings of over 170k and we didn't plan around errant melee strikes on the raid; we figured it's not supposed to happen at all, but only happened rarely so it wasn't worth changing things up for.
Fundamentally, this fight is all on five people; your Magmaw tank has to mitigate as much damage as he possibly can, your three healers have to be up to it and your kiter needs to be on the ball. Tips if you choose to use a hunter would include two things:
1) Traps are your friend. Ours used macros to ensure they were always up when she needed them, but please don't ask what those macros were..
2) Don't try to kill each wave before the next one. By kiting, they WILL die off eventually and you shouldn't feel pressured to kill them - just stop them killing you.
All told, this fight isn't bad at all once you realise what's causing pillars in the melee group and start killing constructs as efficiently as possible. Our composition definitely helps, but I don't think you really need a specific one to complete this. It's more a question of perfecting the plan than perfecting the execution, as there is a bit of leeway for personal errors throughout.
Unwavering Sentinel: Tales of a Protection Warrior Running Wild.