About time to post something I guess.
Basics of position:
Outside Circle - DPS
Middle Circle - Priest [Shadow]
Inner Circle - Cleansers/Healers
Composition:
1 Feral (Stage 1 MT), 1 Prot(Stage 2 MT)
2 Rogues, 1 Enhancement, 1 DPS Warrior
2/3 Mages, 3/2 Warlocks, 1 Moonkin, 2-3 Hunters, 3 Shadow Priest
6-7 Healers, 1 Priest
+Unknown (If not 3 Hunters, or 6 Healers)
Some early theorycrafting on positioning for my guild (The composition was ours so it can be ignored). What has changed since then is that MT healing pallies would typicall stand away from those groups and cleanse themselves when Gas Nova comes (2 second(?) cast) so they don't have to move a lot and can keep the tank up.
The main thing for this fight is that encapsulate forces you to spread out and Gas Nova forces you to group up for mass dispel. The fact that encapsulate has a variable timer makes it harder to accurately call out when to bunch up or spread out (bigwigs could be wrong sometimes).
Mass dispel: Every priest has to make mass dispel their priority since it does 2k+ dmg and drains mana per tick. dispel should be timed and start casting once felmyst casts gas nova.
Corrosion: Tank takes 100% extra dmg with this debuff. MT healers have to always keep an eye and up rank when this is on the tank.
Air phase: When felmyst takes off, the whole raid should be relatively spread out with Pally tank near the middle. Felmyst will target 1 person to cast a green beam that does massive dmg. It can be easily out ran and there's no reason for anyone to die from it. It leaves a poison cloud on the ground (Don't ever run through or stand on it or you get a dot that rapes you). Skeletons will spawn from the poison cloud, healers would usually get aggro. Dps should not try to shoot the skeletons yet until the prot pally has all of them. Range dps can still dps felmyst when he's in the air and it has to be done to beat the enrage timer. There is no aggro reset however and felmyst will chase whoever with highest threat when he lands. Anyone who got aggro on skeletons (healers) has to run to the prot pally. After 2 green beams, you'll have to do something similar to a heigan dance. The map is divided into 4 sections where Felmyst will Deep breathe. If you get hit by it, you will be MC'd with dmg, healing and speed gain. You will have to be killed immediately or you will kill everyone (I think I holy shocked some guy for 5+k with I got mc'd). When dance phase starts, you will have plenty of time to aoe down the skeletons so it's not a rush.
Consumables that help: Arcane Cauldrons. Encapsulate will be the main ability to kill the raid quickly, it's arcane based so arcane pots are very good for this fight.