...but that's no ordinary gas cloud!
Some stream-of-consciousness stuff about Festergut
Oddly I will start this tactics note by discussing healing. Initially raid damage is very high but tank damage is low. This slowly reverses during the fight so that raid damage reduces and tank damage increases to the point where the tanks will need to use cooldowns to survive the boss' normal melee attacks. The fight is also a very hard dps burn with a tight enrage timer. You can see our DPS count in the screenshot in the post below - 5k is the absolute minimum here. You will note the comparatively high tank dps - this is due to a buff which increases the tank's damage considerably and is one of the interesting and fun aspects of this well thought out fight.
Festergut applies a debuff to his melee target every 10 or 15 seconds called "Gastric Bloat". This ability increases the tank's damage by 10% and stacks to 10. If it reaches 10 stacks the tank explodes and kills the raid, so the tanks need to swap at nine stacks. Once you've swapped tanks, the tank who is not tanking (if you follow me) will have +90% damage, so needs to switch stances/forms or turn off righteous fury if they are a paladin. They can then go all out dps for a while (I was pulling about 4.5k dps at this stage) while they wait for their debuff to expire. It's important to note that you must only swap at nine stacks and not before - if you swap earlier than that then by the time you are ready to swap tanks again the first tank's debuff will not have expired, and so you will have one of your tanks with a stack of 10 bloats and therefore a wipe.
The second ability to worry about is called vile gas. It is an AoE and targets outside melee range as priority. In 10 man it is cast on one target at a time, splashing to targets in about 8 yards. You must leave at least one person at range to soak this, and ranged should spread out to avoid it hitting more than one person. It hits pretty hard and applies a disorient effect. If this hits your melee/tanking team it's probably a wipe. Due to the potentially disastrous effect of having a healer disoriented, we ran with three healers, and had two of those healers grouped up with the melee. This meant they were never affected by the gas.
The third point of concern is the gas cloud that permeates the room. This does shadow damage to the raid every couple of seconds. Every so often Fester will suck up a proportion of the gas, reducing the amount of regular shadow damage done to the raid, but gaining a +30% damage buff. He will do this three times. By the time he's sucked up three lots of gas, he will have +90% damage, and will melee the tank for about 25K per hit. It is vital for the tank to chain defensive cooldowns during the time the boss has three inhale stacks.
After he has three inhale stacks, the boss will eventually vomit up all of the gas back into the room, doing about 50k shadow damage to each person in the raid. That is bad, but it can be avoided, as follows. Around the time of each inhale, the boss will cast an infected spore debuff on two people, and the infected person will get a Loetheb-style spore symbol on their head. The spore does AoE damage to the infected person and everyone near them, but when the debuff timer runs out everyone within a certain radius will get an "inoculated" debuff, reducing shadow damage taken by 25%. This stacks three times. The inoculated debuff is removed after the boss vomits.
We used the strategy of all the ranged running to the centre at each spore phase. This lead to high AoE damage for the raid from the spores, but with three healers we healed through it. It ensured that everyone got a chance at being inoculated. Bear in mind that inoculation is a binary magic debuff, so it can be resisted (which is a pain). It's worth having some kind of plan if you are unlucky enough to resist two inoculations in a row (dispersion for shadow priests, mage armour for mages, hand of protection etc).
So, for melee dps this is very easy - stand at the boss and nuke. For healers who are standing next to the melee team, they stand there and heal. For healers standing at a distance and ranged dps, they should run in to the melee hug at the spore phases, and run out afterwards.
For tanks - the worst geared tank should pull the boss and begin tanking. The first tank will get to 9 stacks of the debuff after 2 inhales and once the other tank taunts they should go dps. The better geared tank will then tank the boss through the first 3-inhale phase, and will need to blow cooldowns in this phase. External cooldowns such as guardian spirit may be needed. Due to the very short berserk timer, you should be killing the boss around the time of the second vomit.
I would suggest saving heroism for just after the first tank swap, so that the super-powered tank with +90% damage will get full benefit of the additional dps. I used shattering throw in this phase too.
Pictures of your in-game food
3 hours ago