Play with at least one person and reach level five, then the person with the most points open the stairs (not the help room).

Then one person goes to the grenades on the wall and the pillar closest and run and jump towards the grenades, when in air crouch and try to land under the grenades.

If you do it correctly on your screen it should say "Can't Stand Here" - don't move from under the grenades.

You can turn around and buy grenades if the glitch works.

To see if it works stay under the grenades and see if the Zombies are stopped by an invisible wall.

But beware the Zombies can push each other forward.

You will live, but your friend will keep dying and will respawn if you stay alive through the waves.