In order to get his achievement you will have to fast travel to Minas Morgul and walk towards Mount Doom (You can place a waypoint to the fast travel next to it as a guide).
The achievement should unlock once you cross the bridge next to Cirith Ungol (It is possible for the achievement to pop by going from mount doom and crossing over the
Cirith Ungol bridge and doing it that way)
Video attached for a visual guide by HappyThumbsGaming.