Extinction mode has three total areas that you must progress through to reach the end. You will have to clear 4 hives and then allow the chopper to clear the way allowing you to progress to the city and unlocking "City Dweller." After clearing out more hives, and allowing the chopper to clear the way again, you will progress to the cabin, and will unlock "Cabin Fever." Now, in the last area you will yet again clear out more hives, and then you will have to call in a nuke. Once the nuke is called, everyone will need to run back to the beginning of the level within 4 minutes where the chopper is waiting. Once you escape, you will unlock this achievement.