1. Enable the skull -
Acrophobia
2. Start the mission Exodus on Rally Point Charlie (any difficulty)
3. After finishing the Falcon segment, use the Acrophobia skull ability to fly towards the far left of the map pointing between North West & North on the compass (the middle dot).
4. Using the Acrophobia skull, stick to the cliff side by flying, pass a couple buildings & keep flying all the way until you see a building with the words "New Alexandria" right above it. It's very hard to miss unless you're looking straight down or straight up.
5. Right below those letters is where you want to be, there will be 3 of these weird "tree plant" things. Move around that area until the achievement shows up.