Strategy

Every 20 seconds or so, Festergut will cast Vile Gas on a ranged raid member. This will hit anyone in a 8 yard radius around the initial target, disorienting and leaving a heavy damage DoT ticking on all of you for the duration. Because of this it is very important that every stays around 10 yards from one another. Most modern boss mods such as DBM, BigWigs, and/or DXE contain Proximity windows to help you with this.

Around every 35 seconds, Festergut will mark three people in the raid with Gas Spore. In the original version of this fight, they looked like Hunter's Marks over the heads of people, but this has since been replaced with a Spore-looking icon similar to what you might find facing Loetheb in Naxxramas. After 12 seconds these marked players explode, dealing a small amount of damage but more importantly leaving an Innoculated buff on all who were hit by the explosion. This Innoculation stacks with itself, each stack preventing 25% of the exhale damage that would normally one-shot your entire raid.

When the three people are marked with Gas Spore, one should run on top of the melee and tank while the others form two ranged camps at different points in the room. By doing this, everyone in your raid will receive the Innoculation every time the Gas Spores explode, allowing you to avoid 100% of the exhale damage every time! However, you still have the Vile Gas to worry about. So the moment the Gas Spores explode, immediately race back to your original positions and re-confirm that you're not within 8 yards of one another. You should repeat this every time Gas Spores are thrown out: clump, spread, clump, spread, etc. Melee should always have a Gas Spore running on top of them, so they literally don't have to move at all from the moment you pull the boss.

Between exhales, called Pungent Blight, Festergut will Inhale the gas around the room three times. These Inhales come in 20-30 second intervals, and each one will reduce the raid-wide DoT players have for standing in the gas (unavoidable), as well as increasing the damage and attack speed of Festergut himself. By the third stack of Inhales, and before he exhales it about 20 seconds later, your tank will be hit like a truck. This is the time you will want to chain cooldowns, or if they aren't available yet for your MT, have an OT taunt and pop his or her's. Typically you will only need one tank timing their cooldowns wisely, but it never hurts to be safe.

Repeat the dance of clumping and spreading out, and try to keep everyone alive. In the 10/27 version you have exactly five minutes to kill Festergut before he enrages, and he has around 42 million health. You still have some time after the Enrage to kill him, as he doesn't melee during Inhales which have a cast time, allowing you to sort of kite him around for 30-45 seconds post-enrage.