I tried the other video in the guide, as well as a couple of others on YouTube and couldn't quite get them to work. Here I noticed this would be a perfect place and got it first go without any problems, so just wanted to share the spot with others.
In the story this is immediately after you get the Broadhead Climbing Arrows. If you are past that point then fast travel to the Copper Mill Bridge
camp in the Soviet Installation
area. Use the climbing arrows to climb up the red wall just right of the camp, you'll then be able to walk up a small ramp and use a zipline which will take you to a small platform with a further zipline down. This is the zipline you'll be using, as you get close to the end be prepared to hit
and aim yourself towards the third and final zipline. This should unlock as you slide down.
I'm sorry in advance that the video isn't longer, and starts while I was already on the zipline, but the GameDVR decided that today wasn't the day it was going to let me record a longer clip.