You will obtain the Rocket Launcher in the second mission as you go over the overpass. You will then need to destroy the second Alien Air Unit. After destroying it, continue the mission. You will encounter 2 enemies shortly after; 1 on the roof and 1 on the ground. Kill them and 2 more enemies will appear on the roof. Kill them then pull out your rocket launcher and sprint down the alley. 2 Aliens will come out beside each other. Shoot the rocket right in between them to kill them both. If you didn't kill both of them, click restart checkpoint because you will restart right beside that area. Make sure you do NOT continue forward if you don't get the achievement because you will complete the mission.
Note: If you are going for Color Me Impressed restarting the checkpoint will disable the achievement.
Thanks to Tyger7 for the image.