In survival mode on Dead Center, take the grenade launcher and go to the right flight of stairs.

You will go to the top railing (the one that curves) and crouch. Someone else will stand on top and jump up and down.
The grenade guy needs to shoot his torso, but make sure you are under him.

If you do it correctly he should fly up to the ledge and stay up. It will protect you against special infected.

Beware of spitters and who ever is shooting them up always carrier a defibrillator.