[quote name='Venent']Can this game be completed cooperatively online? I'm stuck on the viking.[/QUOTE]
No online Co-Op for story, only local. You've got the katas by that point, so switch to those then near his head jump and press Y. That should leave him kneeling and open to damage. Or run up the sidewalls, jump, and press Y for some good damage. Avoiding the zombies are the hardest part of that boss fight.