If the tip of his arrow shimmers, he is preparing an Impaler Arrow. Avoid being pinned to the wall at all costs; his Planted will do 2400 damage before you can even move again, and likely another 2400 to follow, killing anyone.
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.