You need to first get a green Triple Rocket power-up, that looks like, you guessed it, three rockets side-by-side. You may or may not unlock this while completing the campaign. Whenever you get one such power-up, try and fly around and gather a bunch of enemies, then shoot into them, one shot at a time. This is not overly difficult, but if you are spamming it may never unlock for you.
A sure-fire method to earn this achievement is to load up Level 30. Shoot blocks around the outside of the level until you get a Triple Rocket power-up, making sure not to disturb the pink tanks in the middle that contain all the enemies. This may take several minutes as this power-up is rare to spawn. Once you get one, unload to break open a bunch of the pink tanks. Then, when there is a large cluster of enemies, shoot the triple rockets into them one shot at a time. Make sure you are quick enough to break the tanks so that the power-up doesn't expire before you get the achievement. You need to kill one enemy with each of the three rockets.