Instructions
Stand near any of the barricades and wait for the zombies to approach. Do not kill them before they get to the barricade.
Allow the zombies to break through the barricade. Once they are through, you can start attacking them at will.
Shoot each zombie two or three times in the body. You get points for simply injuring zombies, so this will give you a head start on points.
Strike each zombie in the head with your knife once you've done your damage with your gun. In addition to the points you receive from shooting them, you get 130 points per headshot with the knife.
Rebuild the barricade once you've killed all of the zombies in this particular wave. Although this seems redundant, you receive points when you rebuild barricades, even if you do it so the zombies can break through it again.
Repeat this strategy until you get to Level 3. Once you reach this level, the zombies will be too strong to kill in this manner, and there is no fast way to earn points after this level.