Run to the nearest armory, get a security guard, grab a pistol and kevlar, and asks him is there a way to get out of there without relying on the elevators or the trams. Hope that your escape route doesn't have hazardous materials on the way out. Once the survivors got up to the surface, hotwire a car and get the fuck out of Horny Mesa, preferably before an army of Michael Tsarouhases turn the entire facility into a long Checkpoint match of Insurgency Sandstorm.
Shoot everything that isn't a human. I don't care how powerful the new X-rays are, Samuel Colt is the great equalizer, and so do his children and rivals. Fingers off the trigger unless spotted a target. Don't point a gun at things you don't wanna kill. Watch what's in front and behind the target. Always remember to treat a weapon as constantly loaded, even though God himself is positively sure that there are no rounds in the chamber. In combat situations, line up a shot fast, brace for recoil, and let loose.
If all else fails, save a bullet to yourself, and a frag grenade if you got resurrected by some magical means.
Taunt the extraterrestrials. Tell them to go fuck themselves and send a few rounds on the way to show them that you mean business. If any rounds actually hit someone, good for you.
Grab a small group of survivors; your friends, some security guard, and even a guy in the HEV popsicle if you can, but limit the numbers. You need enough people to get each other out, but you also need to be mobile and quiet. Gather canned foodstuffs, water, bullets and vests, and ferry them out. There is no sign of the invasion lasting for an hour or so, and no sign of you getting out of there in the afternoon.
And if you somehow find a woman dressed in a business suit, immediately put a round into your head. Your fate is doomed.