I also noticed this. I am playing (well was playing) a Ranger while my coop friend was a warrior. His damage was middle of the road like 8-20 on the first set of zombies. He could do 20-40 on the zombie hogs. While I had a decent bow 8-21 I think. I was hitting the zombies for 3-6. It was no fun so I started learning spells with all my gold. Much better. So I think until they can do something about that Im stuck playing mage!