This achievement is for killing 10 enemies with Rocket Launcher that is unlocked after beating the game. You will want to conserve your ammo for the rocket launcher as it is limited and I have not found a way to replenish it's stock after it has been diminished. Your best bet would be to play chapter 2 and gather all the zombies together at the end of the level and try to take them all out. Again you will want to conserve the ammo because the Rocket and the Magnum are invaluable when it comes to the 5 hour achievement, Chapter 10 achievement for killing Laura and bosses in general. The rocket can also be helpful in a run without upgrades. I would recommend trying for this achievement during that run.
Also keep in mind that an Agony Bolt will reset your progress if you use one before you hit 10.