He's on the left side of the wing, and is the "gas" boss. I'd initially expected a raid-healing-heavy fight, but found instead that most of my healing was pouring into the tank. We used two healers, and spread our ranged dps out across about 1/3 of the room (about 4 players, at least 10 yards spaced from each other) and congregated our 2 tanks, 2 healers, and 2 melee on top of Festergut himself to share AoE heals and spores.
The tanks need to swap the boss between each other at 7-9 stacks of the gastric bloat debuff; the previous tank must stop attacking altogether, as the debuff also increases their own threat via damage, and they might pull aggro back if they keep attacking. Do NOT let this stack reach 10, or you will effectively wipe the raid as your tank explodes.
As a note, due to the intense damage laid on the tank when he gets to three inhales, it has generally been found that any warrior tanks should pull first and tank through the 1st/2nd inhales, allowing a swap to a different tank before the third inhale's heavy damage comes in. Prot pallies or feral druids are a better suggestion for soaking the heavy damage that comes in with the third inhale. The timing of debuff stacks should allow you to swap tanks before the third inhale.
To quote our paladin,
"Prot Pallies are better for the third inhale simply due to Ardent Defender which allows for Damage Redux < 30% and the life save effect on death (Ala Guardian Spirit) which we have used before as a cooldown. (Big Bang on Algalon). Feral Druids are good because they have so much dang health esp with survival instincts up. Very difficult to two shot them between heals."
The spores are visible, graphically, like those from Loatheb, but they attach to their target rather than free-float, so the spored players can move to better position themselves to share the shadow-resistance inoculation the spores will grant everyone in range. Two will spawn in the ten-man version of the raid:
- have one of these move to the melee (if not there already),
- the ranged dps will collapse themselves on top of the other, out in the ranged field (have a spore run over if not there already).
After the spore explodes, spread back out to your places.
Periodically, Festergut will inhale gas from the room to buff his damage and attack speed against the tank (and he will be hitting HARD, particularly after 3 inhales!). Healers and tanks will need to be quick with their cooldowns to ensure the tank's survival as the boss grows in power, using cooldowns as available at 2 and 3 inhales. Resto druids, make use of your Nourish and keep full HoTs rolling.
After three inhales, he will "exhale" in a massive shadow attack against everyone in the room: the reason for the inoculations. An un-inoculated player will be one-shot; the more inoculations on a player, the less damage they will take.
After the raid-wide shadow breath, all of the gas in the room will reset, along with Festergut's damage/speed buff and the inoculations. Rinse and repeat.
Most of the healing will be directed to the tank. Each fresh addition to the tank's gastric bloat stack causes about 10k damage, and each time Festergut inhales he increases his damage and attack speed by 30%, so the tank will be your most-spammed heal target.
If you stand with the melee, you should be able to reach all of the ranged dps and avoid getting hit by the disorient-attack he throws at ranged players. A ranged player will periodically be hit by the vile gas (which splashes, which is why they spread out), and they'll take a bit of damage and become "disoriented" momentarily. It's not difficult to keep the non-tanks up, especially with HoTs.
With a 5-minute enrage timer, you will need to have high dps in the raid; burn him down as quickly as possible while making sure you swap tanks and get inoculated with shadow-resist from the spores. I am not sure if there is a minimum number of people required to be at ranged in order to avoid having the vile gas (aoe on target + disorient) be targeted into the melee, but if there is, I am guessing it will be 3 players akin to General Vexaz in Ulduar. As a note from Sunkist of Barkskinning Up the Wrong Tree, the 25-man version requires 8 people to be at ranged.
Have fun :)