The easiest way to get this achievement is pushing the rocket launcher stormtrooper's rockets back into its face after it shoots them. You just use the force push ability while facing them and it will work great.
If you don't get it during the campaign, you can easily get the rest of the kills you need after you beat the game by going into the meditation training mode and spawning in the rocket launcher troopers. Just spawn in five of them across the arena from you and push back their rockets until you get the achievement.