This works best on 4 player multiplayer.

On the campaign "No Mercy" choose the chapter "Rooftop Finale". Continue through until you get onto the roof.

Once on the roof have one person run on top of the building to your right with the satellite on it.

Note: No one shoot any zombies, only special infected.

The person goes on top of the satellite roof and stands on the corner nearest to the other survivors.

That person then crouches and waits.

Note this person must not shoot any zombies that are below him.

The zombies below him will then bash ending up breaking up the door below him.

Once the door is broken down proceed to killing the zombies.

Once you and your team is ready summon the rescue and quickly run to the room where the zombies broke down the door.

IMPORTANT: Remain at the back of the room and do not shoot or do anything.

Wait until you hear the helicopter.

Note: The zombies, special infected, and even tanks cannot find you.

The tanks will run out of time a disappear also.

Once the helicopter comes it is best if you throw a pipe bomb.

Run like crazy and get on.

If done correctly and quickly you will get the Untouchables achievement.