Upper level/ Pushing the third stone panel - Once you are done killing the Yetis, go around the walkway avoiding Natla's fireballs.

When you reach the gap, take a look above to the right to spot a metal ring. Use the Grapple to catch it and wall run to the opposite side.

Follow the walkway around, avoiding Natla's fireballs. A Yeti thrall appears from the waterfall on the right, so kill it.

When you reach the end, enter the alcove and push the stone panel to reveal the third power mechanism.

(Note - Do you remember the panel you could not push on the lower level because the walkway leading to it was broken? Well, here it is now! Although it is the third mechanism you reveal, it will be the second one you will destroy in just a bit).

Climbing to the platform with the two low walls - As you could see from the cut scene, the mechanism is just above, but you cannot reach it from here. Exit the alcove and follow the walkway back to the metal ring you used.

Once more, use the Grapple to catch it and jump off the edge. Climb up the Grapple line until you are hanging just below the ledge on the right with the Health Potion. Wall run and jump to grab the ledge. Pull up and pick up the Health Potion.

Then climb up the next two ledges to reach a platform with two low walls; one one each side.