Additional information: This is the last "main" mission of the game. As you've probably suspected, you will be presented with A LOT of very difficult jumps. Furthermore, some of the ledges will be very unstable, so you'll have to hurry there. Thankfully, the main puzzle of this level is going to be very obvious and simple. Furthermore, you won't have to fight any monsters.
Start off by listening to a short dialog between Lara and her assistants (#1). Get ready because you'll regain a full control over her movements in a short while. Climb up to the nearest ledge (#2) and start moving towards the left side of the screen. You'll HAVE TO hurry, because the ledge is very unstable. I would recommend pressing the E key often in order to speed up your progress.
A part of the narrow ledge is going to be highlighted. It means that it's very unstable. You'll have to be VERY quick here, because it'll fall down in a few seconds. Like I've said before, it would be best to use the E key (#1). Once you're at the end, climb down to the nearest platform (#2).
You can turn around now. Make a jump towards a nearest ledge. Make a few steps forward. You'll find yourself on a small slipway (#1). As you've probably suspected, you will have to make a jump towards a new platform (wait for the right moment!!). Climb up. Now you will have to reach a narrow ledge. Once you're there, press the F key in order to reach the lower ledge (#2). Now you will have to start moving towards the left side of the screen.
Once you're at the end of this ledge, make a jump towards the second one. You'll have to be very quick about it, because the new ledge is unstable (#1). Just as before, I would recommend that you use the E key frequently. Head on to the next platform. There's a challenging climb ahead of you. Most of the new platforms are very unstable. You will have to keep moving forward (#2). If you won't stop, you should be just fine. :-)
The last platform should allow you to make a successful jump towards the horizontal pole. It shouldn't be a problem to reach a new ledge from here (#1). Now you will have to climb up to the new platform. It may seem as you don't have a place to go. Thankfully, there is a way. Make a double jump towards a small icon. Lara will automatically use the grapple. Now you will have to make a turn. The objective is to see the icicles in front of you (#1).
Start swinging on the rope. The objective is to reach the first icicle safely. Once again, you'll have to hurry. Turn right. The objective is to make a successful jump towards the second icicle (#1). From here, you should be able to reach the last one. Now you will have to make a final jump towards the narrow ledge (#2). Start moving towards the right edge of the screen.
Once you're at the end, make a single jump (#1). Keep moving towards the right side of the screen. You should be able to reach a new platform very soon. Enter the Accurate Aim mode. You will have to start shooting at the ice, because it's blocking your way (#2). Once it's gone, head on to the new cave.
Keep moving forward. You will come across a much larger area (with the destroyed plane) very soon. As you've probably suspected, the objective will be to reach the lowest level. Start off by making a jump towards the nearest ledge (#1). Now you will have to use a large slipway (#2). Make sure that you're that the jumps are very precise. Otherwise Lara will end up dead. You should be able to reach one of the lower platforms without any major problems.
Turn left. There's a horizontal pole here. You'll be able to use it. Now you will have to make a double jump in order to use the grapple (#1). Start swinging towards the opposite platform. Once you're on it, keep moving forward. Lara will notice a small helicopter. You will have to deal with its crew in a few minutes. You might as well ignore it for now. Head on to the big construction (#2). Start pressing the F key in order to reach the ground floor safely.