This requires you to get 50 kills with the Rocket launcher.
If you don't get it by the end of the campaign then i would suggest to go to missions, chapter 2 - mission14, and once in game purchase 2 crates of rockets.
The reason i use that specific mission is :
1) Its only infantry
2) The enemy infantry are close together and they swarm you all together. So i would suggest to gather as many enemy infantry together and let them chase you, once you see around 20 together fire your rocket launcher and rinse and repeat until all the infantry is gone.
If you don't get the achievement right away, repeat the mission and on the second time it will be yours!