Mage tank on the mage, 1-2 tanks on the warlock, 1 "dps-tank" or real tank on the priest, best geared tank on HKM and pull em apart in separate directions. Tank yours near the door to the next are.
Gruul himself is just an annoying fecker, try to move him in the middle and stay alive. Silences hurt but be ready to pop your avoidance trinket if they are badly timed.
Really, the more harder part is having dps who is not stupid enough to stand in caveins or bunch up together before/during shatters.
Oh err... theres plenty of vids out there which can show you exactly what to do.
gl in there

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