With the Treacherous Ground talent, you can prevent Grohk from using his Ghost Walk. With moderate levels of anti-healing, you can potentially kill him even through the healing of his totems.
Make sure either Grohk or his totem is dead before using your ultimate. Give about a second after the totem dies, so that the enemies' CC immunity wears off first. Otherwise, your Ultimate will do nothing when the enemies have CC immunity.