Additional information: This is one of the more challenging missions of the game. It's more difficult than the previous levels. First of all, it's going to be a long mission. Furthermore, there are a lot of difficult fights ahead of you. Most of the opponents will be heavily armed. You will also have to make a lot of precise jumps. You'll probably have some serious problems with some of them. There's also a boss waiting for you at the end of this level. Thankfully, you won't have to kill it. The mission will only require you to avoid most of its attacks.
Start off by watching a short cut-scene (#1) during which it would be wise to listen what the other guys have to say. Lara will also take notice of the nearby fights. It looks like that the guards are being overrun by some unknown thugs. Once you're ready, make a... JUMP! Don't worry, Lara will use the parachute. Be careful, though. You will have to press the movement key (#2) in order to activate it. As a result, you will land on the roof of a building. Get ready!
I would recommend that you use a nearby machine gun right now (#1). Start shooting at the visible enemy soldiers. Make sure you've killed those who were attacking Lara in the first place. You might also consider aiming at the barrels, however this is not necessary. Once you're done, get back to the ground floor. You will be attacked by a second group of soldiers (#2). Make sure that Lara is jumping a lot. Thanks to that, you should be able to avoid most of their attacks.
You might want to take a closer look at the base, however it is not necessary. Head on to the main warehouse building (#1), the same one that was being protected by the enemy soldiers. You will find a single ladder near the entrance (#2). Use it in order to reach the roof.
I would recommend that you use your machine gun right now. Take out a single soldier. Now you will have to make a jump towards the building he was guarding a minute ago (#1). Use your gun again. This time try aiming at the soldiers located on the ground floor. Once most of them are gone, use the rope to slide down to the lower level (#2).
Start shooting at the remaining enemy soldiers. I'd personally recommend aiming at the nearby barrels. Watch out for the guy standing by the heavy machine gun (#1)! It would be wise to take him out as quickly as possible. Once he's gone, use the heavy machine gun to dispose of the remaining enemy units. Don't forget to pick up ammunition and medkits along the way. Head on to the building with a large cross (#2). You will have to enter it now.
An interesting cut-scene will appear on the screen (#1). Lara will save lives of two soldiers. As a result, she will obtain a keycard. You may exit the building now. Start running towards the heavy machine gun. You will have to use it once more, because new soldiers have arrived in the area. Deal with them quickly. You may proceed to the large gate. Use the newly acquired keycard in order to open it (#2).
Be careful. Enemy soldiers will start pushing explosive barrels towards the gate. They will blow up once they've reached their destination. You will have to wait for the right moment to start running towards the new building (#1). Deal with the remaining enemy units. This shouldn't be much of a problem. You must use the nearby door (#2).
A new cut-scene will appear on the screen. As a result, Lara will discover the location of the main lab complex. Ignore the rest of the conversation. You won't have to be in a real hurry to reach the train. Start off by using a nearby ladder (#1). You will find yourself on a roof of this building. Make a jump towards the second ladder. You will have to reach the top of the tower. Make a final jump to grab the rope (#2).
Once you've arrived at the second tower, use the new rope. I would recommend letting it go near the tracks. DON'T even think about killing all of the enemy soldiers (#1). Take out only a few of them. You should also be very careful, because they'll be throwing a lot of grenades at you. Start running towards the train. There's a bike on your left (#2). Use it! Head on to the tunnel. The game will load a new part of the map in just a few seconds.