Go up the ladder to the elevator. There you will see some boxes, but they are not interactive. Pull the switch and go up. At some point the elevator will break and stop. If you stay on it for too long, aliens will attack you from above, so without hesitation go from the elevator to a ledge to prevent aliens from spawning.
On the ledge turn left (facing the direction opposite the elevator) and follow the narrow path to a ladder. Climb it up and head outside. A predator will fire blue energy projectiles at you. Luckily, they are not self-guided and easy to avoid. After that the predator will leave. Follow the way outside until you come to the only door that can be opened.
When the predator will scare you, turn left for a first aid kit. Turn right to continue your mission. You will come to a turn right and a door straight ahead. Behind the door there is some stuff including an armor vest, some ammo in lockers and a minigun ammo in the desk. For those who want more plot, look for a note and a laptop on the desk and a PDA in one of the lockers. Don't mind predator sounds. When finished, pull the switch to the left of the door and walk out.
Turn left and walk until you see an explosion. Two doors are locked, so up the stairs on the right. There you will find another corridor with lightnings in it making it impassable. Turn right and pull the lever draining the power. A PDA lies on the floor if you need it. Walk out and continue until you come to an opening.
Two soldiers will run towards you only to get killed by the predator. Turn right and run through the door. Follow corridors until you are outside again. Climb the ladder down and continue to the opening where two marines were killed. The same predator will attack you, so get prepared. After a few shots he will run away again. Pick up rifles from the dead bodies and open double doors. There is only one door that can be walked through.
Inside there are two doors that can be opened. The door to the left is a dead end, so don't bother. Go ahead and down the path. Don't mind the predator noises and the movement on your radar. You will come to some boxes stashed under a camouflage net. There is a container with a lot of ammo. Now brace yourself. When you walk by the third set of stashed boxes under a net, that predator will attack you from behind.
Boss fight: Predator
The predator will fire his self-guided shoulder cannon. Not only it's impossible to dodge, but it also deals a lot of damage in a big radius. It's next to impossible to beat the boss in a ranged fight, so try to get as close as possible. Then close enough, the predator will take out his combistick and attack in close combat. Take out any ranged non-explosive weapon you've got while constantly backpedaling. You're faster than the predator, so if at some point you will be too far away, he will switch to his shoulder cannon and you're dead. The most optimal strategy here is to make a few steps back and wait for the predator to attack. As he readies his weapon, make a few steps to get out of his reach and repeat the process. And don't forget to shoot, of course.
When the predator is dead, continue walking ahead. Get ready to be attacked by a few aliens. After that continue to the door to finish the level.