Strategy

The single most important concept to be aware of if you're seeing Festergut's enrage go off is the number of players that must stand at range to prevent Vile Gas from being tossed into your melee group. (Just as many of you did against General Vezax or The Deathbringer.)

Here's a refresher of Vile Gas, from Deathkrik's post on this site:
Inflicts a Vile plague in targeted area, inflicting 4k damage every 2 sec for 6 sec. The plague causes the infected targets to vomit uncontrollably inflicting 4k damage to nearby allies.
(Note that he'll do this to 3 targets, and that each of them are stunned for 4 seconds -- so put a healer in this position at your own peril.)

We got the kill with exactly 8 people standing at range, and had no problems with Vile Gas, and therefore believe that 8 is the number of people you must have do this.

The reason it is important to have only the minimum 8 stand at range, is to reduce the number of players that have to move. When ranged players do have to move to get their Innoculation, that movement must be kept to an absolute minimum for the purpose of maximizing their DPS.

Mages can cheat this ahead of the first Pungent Blight by Ice Blocking through it. (I'm not yet sure if this works for a Shadowpriest's Dispersion ability.) So, Mages are great for standing outside because they can limit their need to move greatly. Hunters obviously have no choice but to stand at ranged, as well.

The image below depicts exactly where we have everyone. 'T' is for whomever is tanking Festergut, 'R' is for your 8 designated ranged dps'ers, and 'Co' indicates a collapse point for where each of your 3 players who get the spore need to go. Have someone in your guild who is good at directing traffic call out which spore carrier is going to the left collapse point, which is going right, and who is staying put in the central clump - labeled 'Everyone else'. Of course, don't forget to have the center spore carrier stand close enough to inoculate the Festergut tank.

The 4 ranged dps'ers on each side must run to the corresponding collapse point on their side, and back to their native position asap. Performing this action as smoothly and quickly as possible may make or break your ability to beat the enrage timer. We also found it helpful to use flares or Warlock portals to label where these collapse points are, to make it easier for spore carriers to know where they have to go (thereby eliminating more excess movement).

Be sure all 8 ranged people are spread out to be near their appropriate position ahead of the pull. Bloodlusting (or popping Hero) is best immediately after everyone has gotten back in place after a spore.

You should also bring plenty of Potions of Wild Magic, Potions of Speed and Indestructible Potions for your 2 tanks. Each tank should pop an Indestructible right after Festergut hits 3 inhales. Hopefully it's a different tank the second time Fester goes to 3 inhales. Dps'ers can pop their potions right before the pull, and then a second time in the encounter.