Very unforgiving fight when it comes to making mistakes, 1 mistake and you can call it a wipe.
Festergut does gastric bloat on his main target roughly every 10 seconds in the fight, and at 10 stacks of this debuff you instantly die, this debuff however also gives you a damage increase of 10% per stack. Would you just go for a pure DPS race you would require: 90K+ DPS which is obviously not doable this expansion.
In order to reduce the amount of stacks i gained in the fight i used the following strategy.
Pull him with army of the death and let them take the first stack which he does upon entering combat. For the next stack you will get vile gas on you and the next stack will also be done on either you're bloodworms or army (Vile gas: makes you wander around for 5 seconds and has a side-effect which makes the boss aggro anything else that's present, like bloodworms/ghouls). For the third stack i used a target dummy (craftable by engineers, not the tinker) just before the stack was going to be applied. I let the fourth stack come through and i have 1 stack now. For the fifth stack i summoned my ghoul because the boss will again cast vile gas, and so the ghoul takes the next stack.
I let the next 8 stacks all go through , this now brings me on 9 stacks and a 90% damage increase. All next stacks that would get applied will instantly kill me.
For the next incoming stack i use a card board assassin (engineering tinker,5 min cooldown), and for the last one i use another target dummy( they have a 2 minute cooldown). After that one i HAVE to kill the boss since i have no means left to not get hit by a gastric bloat.
In the kill i ended with an average DPS of 51K, this required full DPS gear (ilvl 393) with everything reforged out of mastery to haste and the hit/expertise cap.
I also used STR flask/ STR food/stam runescroll II/drums of kings and 2 STR pots.