You'll want to do this as soon as you find a place you can rocket multiple aliens, ideally four or more, but no less than three.
1) Save your game (Save A)
2) Rocket enemies
3) Save your game (Save B)
4) Load Save A
5) Repeat from step 2 until the achievement pops.
Without installing the game, it takes about a minute to do the full loop, so expect this to take 10-20 minutes, depending on how many you can rocket at once.