Kill Box Achievement
Kill 20 enemies with the Chopper Gunner in a single run in "Return to Sender."
Right near the start of "Return to Sender", you get to take control of the machine gun attached to the helicopter above. You'll get 3 gunning runs in this level but this is easily doable in the first one with a bit of luck. Take control of the gun when prompted and just hold down and never let go.
Just keep aiming for all the red squares scattered about and hopefully once you've finished the first run, the achievement will unlock. If not, just wait for the 2nd or 3rd pass and you should have no trouble getting the remaining kills. Feel free to restart your checkpoint if it doesn't unlock just before or after you've finished.