In COOP Survival or offline mode, nothing beats an Automatic Peacemaker UP1 (besides an UP2, but I don't have that many gems).
For most other battles, I have found the Prototype to be my worst enemy.
One underrated gun is the one you get for beating all campaign levels (oxbadsomething). It...
Does anyone know if there is a limit to the number of levels you can beat? Is there some kind of boss you get to fight if you beat a certain number of levels?