If there are some mercenaries left on the field, take them out now. The hill is the best spot to fire as it will give you the height advantage (don't forget about the Lurkers mentioned earlier!). When it's done, pick up supplies and take your team to the elevator pictured above.
Follow the linear path towards the second AA gun. There will be some Xenomorphs to take down on the way, and quite a lot of them. Apart from regular soldiers and Lurkers, you may also run into individual Spitters. Eliminating them should obviously be your priority
As soon as you enter the wider area, find some good covers and start firing at mercenaries and aliens alike. Take advantage of the above gallery. Watch out for elite varieties of aliens, as well as the heavily armored soldier. When the battle is over, get on the gallery or choose the low path.
Personally, I recommend the gallery, because it'll be easier to fire at the enemies around the AA gun. See to some supplies in the middle of the site and take a moment to search the area; there is a Dog Tag lying in the ground (Sgt. K. Morales).
Watch out for the enemies keeping guard at the building on the left. Take them out because that's where the new RPG launcher is. Locate it with the motion tracker (there should also be some ammo and a full set or armor in the warehouse), then use the RPG to destroy the second AA gun.