During the last wave of the "Horde" in Act I - Chapter 4: A Few Snags, a single Guardian will spawn marked with a short cutscene. Kill him, and he will drop his weapon where he crashes. Quickly go to his frame, and pull out the Salvo. Shortly afterwards, two more guardians will appear. Use the Salvo heavy weapon to kill one of the Guardians to get the "Stop Hitting Yourself" achievement. If you have any issues, try playing on a lower difficulty. It is also possible to get this achievement in Horde 3.0 mode since Guardians spawn during some waves. You can also pick up the Salvos from the bonus challenge weapon crates to make it easier.