This achievement is unlocked for killing a mimic item cube. The mimic cubes don't start spawning until the second half of the game. This occurs after you have completed four dungeons and they are reset for story reasons. The mimic cubes do not appear to spawn when dropped off by drone but instead must be found hanging around in rooms.
I got this achievement within the first floor after the mid-point of the game in Joblin and they seem to have a decently high spawn rate.