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.
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.