To get this achievement you just need to shoot down 25 targets marked with the red crosshair above them. The easiest way to do this is to play as the defenders and shoot down the A.I. bomber waves that spawn. These are pretty easy to take down, and if you focus on them you should be able to get this achievement pretty fast. You can also easily make progress toward "The Bomber" achievement at the same time, as the AI will not avoid your missiles.