Play Mission 5: Unconfirmed on the Easy difficulty in Co-op mode. Once you reach the final part of the mission in the drilling facility underground, you will find two mongooses on a large square platform with lots of enemies. Have two players get in one mongoose. The driver has a cannon on the front, and the backseat passenger should try using a shotgun (if possible) and throwing lots of grenades (you will have unlimited grenades) behind the mongoose to kill enemies. This can be done with up to four players. Once you get a total of 30 kills between your entire group in the vehicles, you will get the "Maverick" achievement. It should only take a minute or two to get enough kills for the achievement.