Gruul doesn't have phases, he just grows so his physical damage (both his attacks as well as Cave Ins - NOT SHATTER!) gets bigger. That means that the tank and Offtank will take more and more damage as he grows.
There is no tactic for this fight - its like Aran, pure personal performance fight where you really really try not to kill your friends. Healers needs to focus on keeping the MT and soaker (best is a Feral Druid) alive through Silence and range problems (tanks flying to one side, healers out of range in the other side of the cave after Ground Slam). Keeping the MT/OT is the key to this fight, every other raider must do their best to assist the healers after Shatter - bandage, pot, self heal (I hate to see Druids in catform after Shatter, it means they're not paying attention).
Remember that the main problem in this fight is not the Shatter but the randomness of the Ground Slam. People think that Shatter is a problem but if you're positioned correctly Shatter only hits for 300 hp (later it can get to 1000 but that's nothing really - only if you're not close to anyone, ofc).
There are a couple of safe spots on the stones around his cave, if ranged DPS stands on them they don't get flung randomly - less chance to land next to somebody.
After Ground Slam the tank needs to Intercept/Feral Charge Gruul to make sure they are in melee range or somebody else will eat the Hurtful Strike and get 1shotted. Charging him is also a great place to be far from other people and minimize the Shatter damage.
Good luck,
Nesh
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