Easy "Ack Ack" achievement
After getting on the AA Gun in Mission 2: Operation Cobra, shoot down all 20 planes trying to attack your allied tanks to get the "Ack Ack" achievement. They appear in the following order: 3 Right - 3 Middle - 2 Left - 2 Left - 3 Middle - 2 Right - 2 Left - 2 Left - 1 Middle. Restart from the checkpoint if you miss a plane.