The achievement is unlocked after destroying the 255th aerial enemy. You will be operating a flying unit more likely as 2B since this achievement is usually unlocked in normal game play during the first playthrough. There are well over 255 enemies in the aerial segment of the Flooded City story-related game play. So, unless there is a glitch, expect the achievement to be unlocked here.
The game maintains a cumulative count of enemies shot down. Nevertheless, the count is only registered after a save. Accordingly, one cannot intentionally die in the flying segment of the Prologue after killing several dozen aerial enemies and replay this part of the game repeatedly until the 255 count is reached.
During the Missile Supply main story Mission, you will be required to board a flying unit. There is an access point near by so I highly recommend backtracking a short distance and making a save. After making the save, board the flying unit and proceed to destroy as many enemies as you can. Expect to get two achievements as you complete the Missile Supply main story mission. This achievement for shooting down 255 enemies and the following story related achievement for completing the Flooded City:
If either of these achievements does not unlock, a glitch has occurred. Clear the cache. To do so, turn off the console by holding the
on the console for at least ten seconds and then unplug the console for at least 20 seconds. Load the save you made just before boarding the flying unit and play through again. Clearing the cache will most likely remedy the problem and you should earn the two achievements.
Note: This achievement can be bought for 100,000 gold from the special achievement shop available after the third playthrough. Nevertheless, normal game play should unlock the achievement since the flying segments are story related. Failure to get the achievement through normal game play would indicate an achievement glitch that should be remedied by clearing the cache as described above.