Load up the mission "Danger Close" from the mission select screen in the MW3 Campaign menu.
At the beginning of the level there will be a couple of lone targets walking around by themselves. Ensure you are using the "missile" weapon on the gunship you are in. You can verify by looking the bottom right of your screen.
Then shoot and direct the projectile to the lone target. If the blast radius encompasses any other targets, it will void the achievement so, ensure the target is by itself.
Here is my short video guide: