The rest of this mission requires direct fighting. Kill the enemies that dash inside.
As soon as the helicopter starts pounding, sprint up ahead. Around the corner, you run into more enemies.
The helicopter chase ends up in your falling into water. This is the most difficult fragment of this mission. Dive and make it over into the region, where there are several blowholes. On the ice, there are very well-armed mercenaries looking for Lara. There is no point in engaging them in a direct fight.
Swim up to the first blowhole and as soon as the Y icon appears above the head of the enemy, press the button and Lara will pull him into water and kill him. Under no circumstances should you try to climb out of water. Swim around patiently, from one blowhole to another (it is best if they are as far from each other as possible) and check several more time if you can pull anyone underwater.
If not, climb out of water only if the enemies are as far from you as possible, and eliminate them one-by-one. Then, drop back into the water and swim to another blowhole. When there only are two, or three of them left, you can try simultaneous elimination.
Climb down the hidden stairs and smash the wall with the hatchet to make way for yourself.
You will have to use some special arrows. If you aim at the wooden surface, such as the one in the above screenshot, Lara automatically fires a broadhead arrow over which you can climb and reach another one.
Fire two arrows by aiming at both ends of the wooden surface. You will succeed only if you see the white circle around the targeted area, while you are aiming. You cannot drive them into the wood too close to each other, or the second arrow makes the first one fall off.
Jump onto the hard-beaten ground a bit ahead and drive more arrows into the surface opposite. Jump onto the first one. Note that Lara can still use the bow, while standing on an arrow.