This achievement can first be gained on Mission 3: Blood and Rust. You obtain the Salvo Core after your initial fight with several enemy Titans where you can possibly get the 'Close Shave' achievement. After you enter the sewers you'll see a large blue weapon on the ground. Pick it up, it's hard to miss, this is the 'Tone' loadout.
Like the previous Core you had, this needs some hits to charge it up. There are enough Titans in the rest of the chapter to charge it up. Unlike the previous Core, this one requires a lock-on. Using the special ability, (by default set to Y) to send out a beacon will help you lock-on faster. Shoot an enemy Titan down to a small portion of their health and then fire the Salvo. It fires a ton of rockets and can shred through an enemy's health.