Strategy....

The raid should be divided up into groups so that each priest can easily Mass Dispel a certain group of people when Gas Nova is cast. Ideally, people will want to know where their Mass Dispel will be placed so they can move to it when Gas Nova is cast and spread out again afterwards to make escaping an Encapsulated person easier. Felmyst has a large hitbox so melee should be able to easily spread out around and under her as well.

Felmyst Cleaves so no one except the tank should ever be in front of her.


The Pull
Have everyone already in their positions and have a hunter Misdirect Felmyst to the main tank.


Phase 1
About the time Felmyst first hits the main tank the first tick of Noxious Fumes will occur and tick regularly until well into Phase 2. Half or more of the restoration shamans and holy priests will be concerned with raid healing for the duration of this phase.

Once or twice during Phase 1 Felmyst will cast Gas Nova, at which time all priests should Mass Dispel their assigned groups/areas and people should collapse in to ensure Mass Dispel removes their debuff. When done properly, the whole raid should take only one tick of Gas Nova.

Once or twice during Phase 2, Felmyst will cast Encapsulate on a random member of the raid. People need to stay very alert to this by keeping an eye on Felmyst's target and moving quickly away if they are within 20 yards of the person she is casting Encapsulate on. The cast is one second from the time she targets the person to the first tick of 3500 arcane damage on everyone within 20 yards. People who respond quickly should never take more than 2 or 3 ticks of Encapsulate and, when spread out properly, no one should ever take more than 1 or 2 ticks. Encapsulate is hardest to see when it targets the tank because Felmyst will not turn or change targets, so you may want to call out on voice comms if this happens. Healers will need to be very quick to heal the person who gets Encapsulated since they will take 21k damage over 6 seconds. Other people around the Encapsulated person are also in danger since 2 ticks of Encapsulate and 1 tick of Noxious Fumes will do 8k damage in 2 seconds and will kill most raiders whose health is not topped off.

Once or twice during Phase 1, Felmyst will cast Corrosion on the main tank, causing 10k damage and causing him or her to take double physical damage for 10 seconds. Healers assigned to the main tank need to be very wary of this cast because the original 10k damage on top of a double damage melee hit and double damage Cleave can kill a tank in less than a second (and this almost certainly will happen as you learn the fight). With the exception of such very unlucky circumstances out of the raid's control, the main tank healers should not have a problem healing through this additional damage.

After about a minute Felmyst will take off and will not land for about 1.5 minutes.


Phase 2
Until she starts to do Deep Breaths, Felmyst will stay in the air for Phase 2.

As Felmyst takes off, everyone in the raid should begin to spread out to make it easiest for the person who gets targeted by Demonic Vapor to run away from the rest of the raid without getting anyone else hit by the beam. The beam will target one person for 10 seconds and then switch to a new target. It will be cast on three different targets before Felmyst stops using the ability until the next time Phase 2 comes around. A few seconds after the first beam lands, skeletons will begin to spawn from the trail of the beam. A protection paladin should be prepared to start gathering up the skeletons roughly in the middle of the area. Other available tanks should help picking up skeletons and bringing them to the paladin. The skeletons hit for roughly 1-1.5k damage on cloth so when anyone besides a tank gets aggro from a skeleton they can also help bring them to the protection paladin. Once all the skeletons have spawned, the protection paladin may be tanking upwards of 20 skeletons, so healing should be focused on him or her.

After the beam has gone through its three different targets, Felmyst will fly to the east end of the area and prepare to take a deep breath. At this point one person in the raid should be assigned to watch Felmyst and predict where the Green Vapor will fall and tell the raid whether to move north or south so as to avoid it. Predicting which way the raid should move is a hard thing to do and will take practice. To predict where the Green Vapor is going to land, watch Felmyst once she has arrived because she will pause and then face or move north or south in the direction she is going to take her deep breath. These subtle movements are hard to interpret and that is where practice will be necessary. She will then fly directly east to west across the fight area, dropping Green Vapor under her as she flies, and take a second deep breath. She will then return to the east for a third and final deep breath. Often, there is at least some safe space at the very south and very north of the area, but occasionally Green Vapor will completely cover the extreme north or south parts of the area, so there is no safe spot for the whole of this phase and the raid will need to be alert and move as required.

After each deep breath, the raid should move to the edge of the Green Vapor so they can run as quickly as possible to the next safe place when the previous Green Vapor disappears and before the next one lands. All the while the protection paladin will be dragging the skeletons around so healing should be kept up on him or her. Once Felmyst has taken three deep breaths (or between the breaths if possible) it should be safe to AoE down the remaining skeletons in time to get back into the original Phase 1 positions. After three deep breaths Felmyst will land in the west and Phase 1 will start again.

The diagram shows an example of how the Green Vapor portion of Phase 2 could play out. The raid begins at 1 before any of the deep breaths occur. The person assigned to watch Felmyst sees that the first deep breath will land right across the middle of the area (B/green bar) so the entire raid moves to 2. As the first Green Vapor vanishes, the spotter sees that the next deep breath will land across the top of the map (A/red bar). The raid does not have to move at all to avoid it. The third deep breath covers the entire southern portion of the area where the raid now stands (C/blue bar) so they will need to move up to 3 to avoid it. Note that each time the raid moves, it stays as close to the center of the area as possible for maximum mobility.

Threat will not reset and any damage done to Felmyst during Phase 2 will count as threat so it's a good idea to watch threat throughout and Misdirect Felmyst back to the main tank at the beginning of each subsequent Phase 1.
http://www.bosskillers.com/cgi-bin/bbguild/index.cgi?action=view_guide&guide_id=355&sid=vBiH7s7WMb

Once or twice during Phase 1 Felmyst will cast Gas Nova, at which time all priests should Mass Dispel their assigned groups/areas and people should collapse in to ensure Mass Dispel removes their debuff. When done properly, the whole raid should take only one tick of Gas Nova