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Fatboss: https://www.youtube.com/watch?v=XOGH9MNYFdQ
Method: https://www.youtube.com/watch?v=TuNiYdmqD-U
Written guide: http://bit.ly/1zN9caq

Raid Setup

Tanks: 3
You will want three tanks on this fight to properly deal with the debuffs and taunting.
It doesn't matter which classes are used.

Healers: 4-5
We recommend that you use at least 2 classes that has a healing cooldown, such as druid, shaman and holy priest. Every class is viable for this encounter.
A balanced healer setup is always a safe bet for any fight.

DPS: 12-13
Good single target damage is required--use the appropriate single target dps talents.
A good balance between ranged and melee dps is recommended.

Phase 1
Use Bloodlust on the pull and put as much damage into Gruul as possible. Have healers help out with damage as much as possible here. The damage the raid takes will be low at the start, and go up later as the fight progresses.

Tank Gruul in the middle of the room. You will want to have 3 raid markers around him in a triangle. The primary tank will be in one marker, the melee will stand in another, and the third is for ranged dps. It is important that you split the raid evenly here.

If you have less melee, it is recommended to put your hunters and maybe a healer or two in the melee group. However it is important that they stay a bit behind them, so the Cave In will not spawn on the melee.

When Gruul does the first Inferno Slice, you will want one of the off-tanks to stand on the triangle and taunt him. Have the ranged group spread out in the green area to help soak the slice.

For the second one, you will want to have the 3rd tank taunt the boss from the blue marker where the melee stands. Here you will take 3 Inferno Slices, then ranged will take 3 stacks, and finally the melee will take the 2 last ones.

So 1, 5, 6, 7 in ranged group. 2, 3, 4, 8, 9 in the melee group.

Phase 2
After this phase 2 will begin You will want to move closer to the boss, so you have more time to move away from the Overhead Smash. Phase 2 lasts for 30 seconds.

If you have a spare raid cooldown, it can be used here. We recommend using a raid cooldown after the 6th Inferno Slice to make sure that the raid is full HP before Petrifying Slam. After Phase 2, it's just a repeat from the start.

Tanking
Start with your 3rd tank on the boss. The first Inferno Slice will come and your 1st tank should taunt him to the ranged group.
-The 3rd tank taunts him back, and for the next slice, the 1st tank will taunt him to the melee group. He will keep the boss but move to the diamond position.
-Your 2nd tank will taunt him to the melee for 2 slices in a row, with the 1st tank taunting him back all the time.
-After that the 2nd tank should taunt him to the ranged group and he should keep the boss on him but move to diamond.
-3rd tank should then taunt him to the ranged group for 2 slices, and on the 2nd he will keep the boss on him and move to diamond.
-After this, your 1st tank will taunt him to the melee for 2 slices and your 3rd tank will taunt him back in between.

Then the intermission will begin. After it, we will have 9 slices again.
-Tank 2 starts on boss, on slice 1 and 2 tank 3 taunts him quickly to the ranged camp, 2nd time to the melee group, tank 2 taunts him back.
-For slice 3 ,tank 3 taunts him to the melee and he keeps the boss and moves to diamond.
-For slice 4, tank 1 taunts him to melee, tank 3 taunts back.
-For Slice 5, tank 1 taunts him to the ranged and he keeps the boss and moves to diamond.
-For Slice 6 and 7, tank 2 taunts him to the ranged, and on number 7 he will keep it.
-And for the last slices, 8 and 9 tank 3 taunts him to the melee and tank 2 taunts him back. And then intermission will begin and you can start from the beginning.

It's important to have the tank who tanks him currently (not the ones who taunts just quickly for slice) to stand on diamond because people in the 10 yards around him will also take damage.

Class Tips

Restoration Druid
Dream of Cenarius is recommended to help the raid with damage output.
Use Wild Growths for each Inferno Slice.
Barkskin has a 1 minute cooldown, use it!

Healing Priest
The preferred specialization for this encounter is discipline. It's good to know inside your head which groups will soak the next Inferno Slice, so you can pre-shield them with Power Word: Shield.
Have archangel in preparation for the slashes.

Holy Paladin
Use Devotion Aura for the Inferno Slice + Petrifying Slam combination.

Hunter
You can deterrence Inferno Slice to prevent an application of the debuff while at the same time helping mitigate it.


Below i have some links about some problems i found.

2 or 3 tanks: http://www.mmo-champion.com/threads/1734882-2-or-3-tank-mythic-gruul
Warlock: http://www.mmo-champion.com/threads/1732431-Warlock-pet-on-Mythic-Gruul
Paladin tank: http://www.mmo-champion.com/threads/1727258-Mythic-Gruul-2-tanking-help-me-be-a-better-MT-while-raid-leading
Warrior tank: http://www.mmo-champion.com/threads/1719080-Mythic-Gruul-Tanking

Regards prinsy