On Rooftop finale, kill all your team-mates for this to be easier.

After killing them go outside and go to the building with the satellite dish on it

Once you get there you must go to the edge with the ramp and door under it and kneel on the ledge.

After you have finished that look down at the door. The door is impossible to break open unless an infected breaks it.

After sitting there for a few seconds infected should be trying to hit you.

If they climb up to you (which they will) just hit them.

One will open the the door and then kill em all.

If you go in all the way to the back no infected or even tanks can get in.

This is very useful for getting achievements.