If I were you I would try to convince my Guild Leader to be MT-ing Gruul himself, while letting a Druid OT him. With the amount of spell damage you have on your level, you will enable your DPS to go all out on Gruul. Considering Gruul has an indirect "enrage timer" (due to his Growth), this will give more added value to the raid, than when you are OT-ing, which is a pain, because you need to go nuts on all your threat boosting trinkets, abilities and mana potions to keep yourself away from going OOM. A druid has a far more easy job on maintaining the second spot on the list than you.
Oh, and don't let them uncourage you by saying that Gruul's Silence ability will eat you alive, because I have yet to encounter the first time when my Cool Down is there on the same time as his Silence.
For the rest if you are going to MT him:
* tank him in the middle;
* keep your Holy Shield rotations compact, without any margin for a crushing blow (otherwise you will get problems with silence);
* move away from the Cave-ins, especially after growth 8-11 (you have to figure this out by yourself);
* let the melee DPS know that you need to be the first back into the fight after the ground slam;
* spread out on the Ground Slam;
* buff up to the maximum: buffs, foods, pots, flasks, nightmare seeds, etc.
If you are still going to OT him, just follow the advice from the post above.
Have fun and Good Luck!
Regards,
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