First start the zombie level. Next but the Kar98k. It helps when you do this on co-op. That way you have a guard when you get a new weapon. Kill zombies until you have at least 1950 points. When you atleast have that many points open the door that should say help. Trade your pistol for a random weapon. It might take a while but try and get a ray gun. It kills zombie in 1 hit. Be careful it takes a while for the ry to hit them and it has a small wave where it hits. When you get 1000 points in the 2nd room clear the debris. After you do that go up the steps and
make a U turn. go to the end and kill them as they are running or walking at you. If you have someone helping you have them wait at the window and kill them as they try and break through. I got to level 19 by doing that.