KNOCK THREE SKELETONS OFF THEIR POLES INTO THE RIVER
This challenge is accessible during Chapter 6 - Return to Hidden City. After Unuratu and Dominguez argue and you exit the temple, you have to climb the cliffs to make it to Unuratu's cell.
Along the way, you can find three skeletons - the first is located below the beam that breaks in two, and you have to perform a wall-run to knock it down.
Keep an eye out during the climb for the other two to complete the challenge.
The first rappel point, before wall-running to the craggy wall on the right.
After scaling the craggy surface that crumbles, and before swinging to the plateau.
Underneath the overhang before climbing up to Unuratu's cell.