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.
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.