This achievement unlocks after completing all optional missions set at the courthouse.
1. "The Law"
2. "Civic Duty"
3. "Trouble at the Courthouse"
You can take your time starting "The Law", but be careful, as once you receive a distress call from the survivors at the courthouse (Trouble at the Courthouse) there is limited time to respond. If ignored, zombies will overrun the courthouse and kill everyone, taking the last mission off the map forever.