There are three outposts in each city.
Simply go to the mission start point and complete the mission to unlock the achievement.
It's also very wise to hunt down worms and uncover all weaknesses of all Captain's before you start these missions.
You can also narrow things down to save yourself time.
Go to the map screen and chose the mission of the Outpost you want to destroy. Highlight it and it'll also highlight the Captain guarding it. Have a look on the army screen to see which one it is and when you interrogate a worm. Make sure it's them you choose to reveal.