After the "Star Pupil" achievement this will be the next one you unlock. Simply complete the first mission and this will unlock. The first mission is very easy to do as the only enemies that will be sent at you are dogs and peacekeepers. Simply build enough of an army as possible including vehicles and on foot and then head towards the top left corner of the map and destroy all buildings. I did this using about 30 units. You might end up losing the majority of them in the course of attack but simply send up a second batch and you will definately get the job done then.