Get rid of the third AA gun in mission 10 and proceed through the rocky exterior to a destroyed building. You will find ammo to the right of the entrance, and a destroyed floor past that point. Stop prior to exiting this building. You will see a sheer drop past the exit that leads to a big 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. Stay 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 ammo, then reload the last checkpoint and repeat the process.