Destroy the third AA gun in mission 10, the continue through the rocky exterior to a destroyed building. You will find ammunition to the right of the entrance, and a destroyed floor past that point. Stop just before exiting this building. You will see a sheer drop past the exit that leads to a large open area. From this position at the exit, look to the far right with a scoped weapon to see aliens spawning above the spotlight. They will then run across the roof before jumping down. Remain inside the building and do not drop down into the open area and those aliens will always respawn. Kill them from the building until you need more ammunition. Then, reload the last checkpoint and repeat the process.
