The mechanics of this encounter follow this pattern:
1 Spores appear, (I’ll explain their meaning in a moment)
2 Festergut inhales orange gas and becomes 30% stronger and larger.

Steps 1 and 2 repeat in cycles of 3 . On the 3rd repeat, after the 3rd inhale Festergut does 90% more damage and then casts a spell called Pungent Blight. You’ll see it coming because the room will be free of orange gas and Festergut will be largest in size. After the spell is cast Festergut goes back to his normal size and the process starts again. This spell (Pungent Blight) is very dangerous as it normally does 50k mass aoe damage to everyone in raid. And that is where the spores come in, the spores make you take less damage Pungent Blight.

Have a spore explode next to you and you will take 25% less damage, have 2 spores explode and you will take 50% less damage, have 3 spores explode and you will take 75% less damage from the mass aoe. Managing the spores is the key to this encounter. Ideally you will want to have 3 spores on everyone, but classes with 25k or more hp can survive with just 2. You can realize how many spores you’ve taken by checking out your debuffs.

You will get a message on your screen saying “Festergut Releases gas Spores”. Look around and see who has them, look at your debuffs if you are bunched up in the melee group, maybe you have it and you are not noticing. The raid has 12 seconds to manage 3 spores. 1 for the ranged group, 1 for the tank and melee group and 1 running away from everyone else.

That is why you should always check not to have 2 spores exploding next to each other. Alternatively, if you think it’s a waste to have 1 spore run off, you could have 2 ranged groups set up.

Why have 1 running away? Because the spores do damage, dot aoe damage, have 2 spores go off simultaneously next to the tank or some dps and they might die.

When the spores appear the melee spore runs on top of the tank, all the other melee follow and get bunched up, the ranged spore goes towards the middle of the ranged group . All the ranged get bunched up. Once the spore explodes the ranged need to spread out again because of that aoe damage, Vile Gas it’s called . The melee need to keep switching between the front of the boss for the spore and the back of the to avoid parry.

What about the healers? Healers that are assigned to the tank and to the melee naturally stay close to them and move as they move. Healers assigned to ranged stay in the ranged group.

Since the spores are randomly casted on players, you can’t predict in which group a spore will land, thus you can’t really make rules for ranged or melee, for healers or for dps. You do have 12 seconds to react though, so usually just go to the tank if you are melee, go to the ranged if you are ranged. If you see a spore really close to you check if the other group has a spore, if they also have a spore start running away from everyone else.

Tanks need to watch out for a couple of things. 1st of all a debuff called Gastric Bloat. 10 stacks of this and the tank explodes. So at 9 the other tank should taunt. This debuff also increases the tanks damage by 10%, so the tank, after being taunted off, should switch stance, presence or go to cat form and start dpsing.

Also, after the 3rd inhale the boss starts doing serious damage, so that’s a good time for tanks to start using defensive cooldowns. Pop bloodlust/heroism the 2nd time Festergut gets 3 stacks.

Once the spores mechanics are learned tanks and raiders will stop dying from them. After that is done, you should steadily reach the enrage timer, most likely, at least in the begging you will die because of insufficient dps.