If you get a chopper gunner or AC130, go by the pipe where it leaves the map by the building with the high post (SW corner).

When there, go to the corner and use chopper gunner/AC130.

The enemies will spawn on the top left corner (NE corner) by the 4 huge pipe in a cluster.

While you're in the corner, keep firing right there and you should get alot of kills.

This will most likely only work if that is the spawning area. Works perfect for offline splitscreen!