i believe all you have to do is drop down from a zip line with
, and land on another zip line. here's where i did it:
starting at the Sheltered Ridge base camp in the Soviet Installation, drop down from the ledge, pass the cave on your left, and climb up the little stone ledges to the spot with two trees and a metal telephone pole. climb the climbable tree, jump to the tree stand in the other tree, and climb up higher, jump back to the first tree and onto the stone ledge with the zip line. this zip line has another zip line that ends pretty close to right below it.
if you drop down and catch onto the other one, it should pop the achievement. the first time i did this, i believe it was another location, and it did not trigger the achievement. this time i actually just snagged the zip line for a second and then immediately fell and it worked.
if you catch the second line with your hands instead of your axe, don't drop down, just try climbing back up a bit, then pressing
to slide back down, and it may cause the achievement to trigger. ( credit: MasterOrtega