If Grohk uses Spirit's Domain, his lightning doesn't hit through allies, but his Shock Pulse does. He may try to stay in the front if they group to get some damage in, so use that opportunity to burst him down.
A Grohk with Maelstrom sacrifices healing for large amounts os AoE damage and self-healing, allowing him to solo duos and even trios. Grab Cauterize to shut him down instantly.