Easily obtainable on the first mission, Phoney War.
First, you must have previously played through Phoney War once before to unlock the Tomahawk plane. It has 2x 250 kg bombs. Play the level again like normal. After you destroy the first ME 109 flight, you will be told to follow Cliff throw a valley. You will come across three convoys of ground vehicles. Notice on your screen that there is a little crosshair that moves in front of your plane on the ground. When this crosshair is close to or on top of a vehicle, hit the left trigger (I always hit it twice, for safety) and you will unlock the achievement as soon as the bombs kill a vehicle.
Best done in conjunction with