This achievement requires you to score 100,000.
Make sure to set your sensitivity right for this. Start the game getting to wave 9 without losing a single city. At the beginning of wave 10, shoot the oncoming missiles from left to right on the left turret, continue doing this for the beginning of each wave, then go right to left for the next wave. If any missiles get past you, use the center turret to take them out. In order to deal with the Smart Bombs, you either hit it directly, or lay two missiles a little apart from each other on both sides of the Smart Bomb. Make sure to take out the spaceships as fast as possible. As you try and try, you will get to 100,000 points and it will unlock.