In Trap Mode, UFO's occasionally will fly over you. They're easy to identify because you can hear (and sometimes see) the lasers they're shooting before they appear. Just keep shooting them. You get a few each time you play the mode and the achievement is unlocked when you've cumulatively destroyed 50. You'll get an update notification every 5th one you shoot.
Note that the achievement will not unlock until you EXIT the game, so don't be concerned if it doesn't immediately pop upon shooting the final one.