Lev 15 HaHaHA. Went last night with lev 34 Druid. I'm a Lev 24 Chanter. The 2 Shammys would not add if the Druid attacked Grarr, but they kept him healed.
We tried to attack all three and mezz and root. No way. They major resisted even with tash and they cast 5 different dots on me before I died. I had a MR buff going as well. There is a Gorilla in a "stable" outside and it was attacking through the walls. Totally out of control.
Best and easiest way to do this is to harmony and pull each shammy individually out of hut and fight them as singles. Must be fast to get second shammy and then to go back in to kill Grarr in room. Shammys will respawn pretty fast.
If you buff well before and have breeze and full mana, this will work well and probably at lower levels. Don't go in trying to kill them in the room. Big hassle.
Oh BTW the other mobs will not aggro on you (they were green to me) so the ones that you have to pass going in and out are safe.
Good Luck
Paradiseous Mindmaster