He did:
Mannoroth was pretty much toying with Grom in that cinematic and oops, Grom proved far stronger than Mannoroth gave him credit for.
If the Pit Lord were actually considering him as a serious threat and opponent rather than an annoying pawn, he'd own Hellscream with his magic, easily. He pointed at an Elven sorcerer, said one incantation, and said sorcerer died a very, very painful death.