Invisible Wall
While playing with at least one teammate, reach the fifth round and then have the person with the most points open the pathway upstairs. You should at this point see the outline of grenades on the wall. Sprint towards the outlined grenades and jump; while in the air press the crouch button. If done correctly you will land under the grenades and see the message "Can't Stand Here" on your screen. There will be an invisible wall the zombies cannot pass through but which you can shoot and throw grenades through. Your teammate will not be able join you here since there is not enough room for both of you, but he can still play and if he dies he will respawn at the start of each new round.
Invisible Wall 2
There is another invisible wall glitch/cheat you can take advantage of. It can be done either solo or with a partner, but it is easier if you have teammates. After collecting better guns from the random box in the help room, open the pathway upstairs and find the small piece of wood leaning next to a cabinet in the corner of the room. Sprint towards the piece of wood and jump, crouching while you are landing, then immediately hold B to go into prone position and then crouch once more. If done correctly, there will be an invisible wall that the zombies can not pass through and your teammate can cover the single window behind you. This will make it so the zombies will never be able to touch you, as long as the window behind you is secure, you will survive many rounds. Again, only one person can do this but since you will be attracting all the attention from the zombies in front your teammate will remain safe.
More Points
Getting the most points possible early on is crucial to success later on. The more points you have the better the guns you can get. To get the most points possible in the early rounds, shoot the zombies three to four times and then use your knife to finish them off when they get close. This will give you points every time you shoot then and give you extra points for using your knife.
Save the Last Zombie
You will be given a few seconds after you finish off the last zombie before the start of the next round. However, you should not kill the last zombie so quickly. Letting the last zombie, preferably the slowest one, live will give you extra time to board up the windows and walls and more tries at the random box for a better gun. This works best with a partner to distract the zombie but you can do this while playing alone.