lol, I never had a problem with gromnir + friends. Of course, I DID have a Keldorn, Saervok and my character as a half orc barbarian, but whos checking?
If you have lots of fighters (3 or so), keep 1 back to slow down the small army of punks that attack from behind. Send the other fighters to kill gromnir and mages. Use clerics and mages to assist.