The task is simpler that it may seem at first, because the Xenomorphs will be much more interested in attacking the main character and O'Neal instead of Reid working at the comm tower. Since they show up in packs, I recommend using explosives more often than usual. It's also good to shoot them from a greater distance, especially given that they'll only be coming from one direction.
The battle will come to an end when the overgrown Xenomorph that cocooned you earlier you shows up. Don't waste ammo trying to shoot it; it's immune to your attacks. Wait until it starts chasing after Bella and Reid, that's when you'll receive new orders.