Grobbulus has always been a bit of a pushover and has always been regarded as one of the easiest fights in the zone. But there are a few important mechanics that the raid needs to be aware of.
Every 15 seconds he drops a poison cloud where he is standing that deals a bunch of nature damage and persists for 75 seconds. The tank needs to walk the boss around the room so that the melee can dps without standing in a poison cloud. But the tank has to minimize movement or else the long duration of the poison cloud will mean that there is no safe place to stand in the room.
Grobbulus has a frontal cone attack called Slime Spray that deals a lot of nature damage. Anyone hit by the spray summon an add called Fallout Slime which does aoe damage and should be picked up by an offtank. If all the ranged/healers are in the center of the room and the tank is walking the boss around the outside of the room and melee is all behind the boss, then the MT is the only person who will get hit with Slime Spray which means there shouldn’t be too many Fallout Slimes to worry about. A few ranged can kill the Fallout Slimes relatively quickly when they spawn.
The boss has another ability called Mutating Injection. Every 20 seconds or so he will cast this on a random person in the raid and it adds a debuff (and possibly a DoT?) that lasts for 10 seconds. At the end of those 10 seconds the person explodes and does a whole bunch of damage to everyone around them and they drop a poison cloud where they are standing. The debuff is cleansable and that is where groups might run into trouble. When the debuff is cleansed, the person doesn’t explode, but they will drop a poison cloud on the spot they are standing. Likewise if a mage tried to Iceblock off the debuff or a pally tries to bubble it off. There needs to be 1 person responsible for cleansing and nobody else should even think about cleansing. When someone gets the debuff, they need to run to an area where there is already a poison cloud on the ground and only then will they get cleansed. By doing that, the poison cloud that drops when the person gets cleansed is in a spot where there is already poison on the ground so you don’t lose any free space.
The fight is really about managing space. If the tank is moving the boss too far between poison clouds and you have a few cleanses in bad places, the whole room will be full of poison clouds and the raid wide damage will go through the roof and you will probably wipe.