This glitch can either be done solo, or co-op with friends.

Firstly stay with the first room until each person has enough money to open the random box as many times (if playing solo, open the "Help" room when you have a lot of money, or if there are too many zombies within the first room).

It is necessary for the box to opened as many times as possible as at least one person needs to have the flamethrower, as this has unlimited ammo, therefore meaning that you can stay in the glitch for as long as you want.

Once each person has a good weapon, (if co-op at least one person needs a flamethower, if solo the flamethrower isn't necessary but you WILL run out of ammo eventually).

Then open the upstairs area from within the "help" room, once up stairs the glitch is beside the cabinet containing the scoped sniper rifle, if you look at the pillar beside the cabinet, then to the long broken wall, you will notice that there is a small area sticking out from the wall, next to this is what looks like a square piece of wood leaning on the wall.

To perform the glitch simply sprint and jump so that you land on the wood, then lay prone and turn 180 degrees so that you are facing the stairs leading up from the "help" room then go from prone to crouch, then you are within the glitch and therefore cant get killed.

If you are playing co-op, then one person does the glitch preferably with the flamethower then the other players need to stay by the window behind the question mark on the floor, they have to stay behind it and cover the window, but if you are solo it doesn't matter if they come in through the window as you still cannot die. To check that you are within the glitch simply let a few zombies come towards you.

They will walk towards you and stop about a meter in front of you, but they cannot reach you they do step forward, but are still unable to get you, from there you can stay as long as you want.

I did this glitch, and it does work as I got to level 66, but gave up as it took two and a half hours!

Hope this works, and you enjoy it!