Mission: Act I Chapter 4: A Few Snags
During the last wave of the "Horde", a single Guardian will spawn marked with a short cinematic. Take him out, and he will drop his weapon where he crashes. Quickly go to his frame, and pull out the Salvo. Shortly after, 2 more guardians will spawn. Use this Salvo heavy weapon to kill one of other guardians to unlock the achievement.
You may have to play on a lower difficulty if you are struggling.
It is also possible to attain this achievement in Horde 3.0 mode as guardians do spawn during some waves. You can also pick up the Salvos from the bonus challenge weapon crates to make it easier.