Make your way towards the entrance to the above building. Keep your eyes open, because the place will soon be overrun by Xenomorphs, Lurkers among them. When all of them are dead, head for the exit.
The next part or the mission will have you run through an area full of containers. As you may be guessing, the aliens won't give you a break, so be on you guard, don't let them get too close and kill the elite ones as fast as you can. Watch your back; not every Xeno will show up in front of you.
When you pass the last container, you'll reach an area which is also full of Weyland-Yutani mercenaries. Most of them are busy with the evacuation, though you shouldn't ignore them; some of them may still turn against you.
Take the above pathway towards the ship outlined in the distance. Clear your way of mercenaries (the grenade launcher should be helpful) and aliens.
When the ship flies away prematurely, focus only on getting to the dropship piloted by Reid; this will end the mission.