Grover's ultimate does not make him CC immune but stunning him will not cancel the aoe heal. Displacement heavy comps are very powerful against him since they can keep him away from his team and reduce his healing.
Assuming Buck hits you at point blank with all his pellets in the head, he will need 3 shots to kill you.(2.8s) This will never happen. You can knock buck down to 500 HP with the Daggers+Pounce+Daggers combo (about 2s). Easy kill from there.