Here is a guide for 11 miscellaneous achievements around Paradise Killer, with this one being the seventh one.
1. You have to scale the massive pyramid above the southern beach. Note, you will need the starlight upgrades first.
2. At the top, in the fenced in area, is something to hack. It will gas the barracks.
3. Climb the barracks, which is it's own area near the start of the game, that is closed off. You have to climb the ladder on the right and scale it.
4. On the very top is a trap door. Open it, and go in.