Follow your teammates towards the entrance pictured above. Be on alert because as soon as you go inside you'll be attacked by a group of Lurkers. Take proper precautions in killing them.
Another group of Xenos, though not too large, will be waiting for you after you leave the building. Notice some mercenaries defending themselves in the above valley. Theoretically, you could wait until they're taken care of by the aliens, but it's better to just get rid of them yourself. I suggest showering them with grenades and then shooting the rest, taking advantage of the height difference.
Go down when the fighting is over. Pick up collectibles and make sure to disable a sentry turret. Make your way towards the third AA gun. When you're there (the above screenshot), eliminate a group of Weyland-Yutani mercenaries. Use the scope to aim at the explosive barrels.
Destroy the third gun the same as the other two, with the RPG launcher. You'll find it inside the container pictured on screenshot 1, but make sure to pick up the last Legendary Weapon first - Frost's Flamethrower.
Get the RGP launcher (you'll have to drop the flamethrower first) and use it the only way you know how to destroy the third and the last AA gun.