If you have downloaded this map Der Ries, than you should get help from these tips (Co-op works the best).

On level 1-2 dont use your pistol ammo, knife, it goes way faster.

Then when both of you get at least 2500 points open the 2 doors that are to the right of the Main Frame. Then one of you opens the door that is worth 1000 points and the other goes to the mystery box but dont open it.

Wait until your partner gets to the power switch and then one person turns on the power at the same time the guy at the mystery box opens it and you usually get a good weapon or monkeys.

Then when you get to the dogs stay back. After that stay in that room until you get good weapons such as Ppsh, MG42, Ray Gun, and Wonderwaffle and get a good amount of points (2000).

When the last guy come get him on the ground by throwing a grenade at him or by him. Next try to link all the teleporters.

One person links them and the other person just stays at the main frame to link it right away. When one person runs out of money then just switch jobs.

You need to get to the last teleporters in the very back and go underground to shoot the zombies. When you get 5000 points get a guy on the ground and go to the mainframe and put one of your weapons in the pack of punch machine and continue to get 5000 points, just putting different weapons in the pack a punch machine until you get the weapons that you like.

This should get you to level 20 or higher if you're smart with choice of weapons.