The developer level is a dlc pack!
dlc packs can be very confusing to open the first time you play them. If you follow these simple instructions you'll have the achievement.
1. As soon as you're on the main menu click chapters
2. as soon as you're on chapters there is a button next to X that says "new missions" (press X)
3. The developer dlc is the 6th pack in the list select it
4. The easiest way to get this achievement is to take a wired micro-usb cable and connect to your Xbox one and controller so it doesn't go idle when your controller disconnects. Just leave your controller alone for about an hour and then come back to the console and you'll have the achievement.