If you wonder about this!
Minor spoiler alert, very minor about location to describe were you get this.
You will see two rocket guys shooting at you while fighting of a few enemies on the ground.
After killing the ones on the ground (To make it easier) just deflect one of the rockets back at one of the two guys and he will fall down and it will trigger this achievement.
To deflect you timely press LT to bounce back their rockets.