Festergut is a DPS race. He’s got a 5-minute enrage timer that you have to beat.
Again, for melee DPS this is mostly a tank and spank fight. Stand behind the boss and hack away.
Aside from the 5-minute timer, the other gimmick to the fight is Pungent Blight, an AOE spell that will one-shot everyone in the room. Surviving the ability will depend on how many stacks of Inoculated you have on you.
Festergut will put Gas Spores on random raid members (2 on 10-man, 3 on 25) prior to casting Pungent Blight. The spores cause a 12-second debuff and then explode. Anyone nearby will get Inoculated eventually (it causes another debuff first). He will cast the spores three times, which means you can get Inoculated three times, before casting Pungent Blight.
As you can tell by now, spreading the spores is vital to the raid. On 10-man, you’ll have a ranged group and a melee group. DBM will mark the two members who get the spores with skull and X. Your raid leader will likely designate which mark stays with melee and which mark runs to ranged. Pay attention to that part because you want to know when you’ll be running to ranged if you get the spore on you. For example, if your RL says X goes to ranged and you get marked X, then you’ll have to stop DPS, get to the ranged group and stay there until the spore explodes. You have 12 seconds to do all that. On 25, it’s the same, except that there are 3 spores and DBM marks the third raid member with square.
Still on the spores, the last thing to remember is that a tank can get a Gas Spore as well. This can screw up your assignments. Imagine the same scenario above where X runs to ranged. What if the tank is marked X? Obviously, he can’t go to the ranged group since he’s tanking the boss. Be ready to adjust accordingly.
Finally, Feint is your friend. When the boss starts casting Pungent Blight, get Feint up before he finishes and you’ll get hit with less damage. It’s even possible to survive with only two stacks of Inoculated because of it. And no, Cloak doesn’t work. Not in my experience anyway.
Get 3 stacks of Inoculated. Bring the spores to the correct group when you get them. Use Feint to minimize the damage from Pungent Blight.