Grobbulus is the second boss you will encounter in the Construct wing in naxxramas.

Abilitys

Grobbulus

Places a poison cloud in a 10 yards wide. The cloud deals high amounts of Nature damage, meaning the MT needs to keep moving while tanking. The poison cloud remains for a very long time.

Hits for a fair amount of Nature damage. Raid members who get hit will spawn a Fallout Slime.

A blob add that deals damage every 2sec, in a 10 yard range AoE.

Injects a target with a mutagen that will do physical damage AoE after 10 sec, or a lower-damage AoE when cleansed. The target will create a poison cloud with the same effect as Grobbulus himself, so the target will need to run to the side when cleansed to avoid damaging the rest of the raid.

An AoE DoT that deals Nature damage over 3 sec. This ability is triggered when the MT gets out of the melee range of Grobbulus (this may occur when MT is passing a corner and adjusting his position).

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Raid Setup

Tanks: 2
Healers: 2-3
Dps: 5-6
Misc: -None-

Tanks: 2
Healers: 7-8 minimum
Dps: 15-16, depending on how meny healers you bring.
Misc: -None-

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Strategy

The maintank stands upon the ramp which Grobbulus patrols, slowly moving him to avoid his poison cloud attack. The MT hugs the outside wall all the way around the room. Grobbulus should be moved as slowly as possible for the best use of space, as his cloud will limit the room's safe zone.

DPS on Grobbulus is ranged, so as to avoid spawning multiple fallout slimes. Only one slime should be spawned at a time(the MT's) which will presumably aggro onto a healer, and should otherwise be pulled to melee classes. They dispatch this slime, away from the ranged/healers, then return to adding ranged dps.

Injected people should run to the edge of existing clouds, as far away from the raid as possible to best use the space. At 30% Grobbulus injects people more frequently and safe space can become scarce if it is not used well from the beginning.

Everyone always needs to be aware of the growing green clouds at all times.

NOTE: No Numbers on health or damage of the abiliys are posted, simply cause most guides havent changed that from the old Naxx, the abilitys are more or less the same but the damage from them have obviously changed.