You need to climb to the top of the tower. It does not require any special skills of you, simply follow the object smeared with white paint.
However, you need to watch out while jumping onto the ice wall. One of its fragments will slide partially, and you need to watch out to jab the hatchet into it again.
Go up and jump to reach the shriveled cross on top. After you start pulling up, you will be attacked by a Deathless from below.
To prevent falling down,, try to aim your pistol at the head of the Deathless. After you take several accurate shots, Lara should eliminate him.