This cheat is done with 4 players, 12 grenades, 1 crawler(without legs).

What you do is open up 2 portals, so the door is half opened,

To get in when the door is like this, 3 of your comrades will have to die by grenades and a crawler(optional).

Have 3 people stand near the door after being half opened, all 3 players throw grenades at the ground below them, if this is done correctly they will be on Last Stand.

The 4th person will have to bring in a crawler, and lead him to the dead bodies, when the 4th player is attacked ONCE he must jump and throw his grenades.

This should make the player float on the dead players, when the 4th player is floating he must go prone (lay down) and crawl through the opening.

Through here you can upgrade your gun, and survive all 100 rounds.