Load up Act 2 – Mission 2: Return to Sender.
Set the game on 'Recruit' difficulty and make sure not to kill anybody!
As soon as you take control of the character, run straight and then right in to a path that has buildings on either side (this is the natural path to progress the mission).
At this point more enemies will spawn as you have moved further in to the level which makes the achievement easier. As soon as you have ran around this area for a few seconds, find a corner to hide in inside a building.
Use your air support (Right on the D–Pad). Don't bother zooming and kill as many enemies as quickly as you can.
The achievement will pop as soon as you get 20 kills, so if you don't get it by the time the air support ends, reload your checkpoint.