The co-op update for the Arena is in beta! You'll also keep the credits from the Arena for the main game as the score and the credits were separated. The high levels now introduce enemy variants with a lots more hp, speed, and even shields.
In addition the game uses a new shader system. This may be a source for some bugs.
You can head on to the Arena forums for feedback on the beta: http://steamcommunity.com/app/428750/discussions/2/
After we've found and solved any major bugs, we'll push this to default branch plus Mac and Linux.
Please back up your profile before running the beta, as it may have some bugs.