Hightower Coal is a good place to get this achievement, but I think I've discovered a much better place.
I found that it's easier to shot back to enemies equiped with rifles (those that appear with the symbol of a target above their heads). They are much slower aiming than the regular ones, so we have more time to tap
two times. And it's even better if we get close to them.Buckingham Palace's roof
has four guards equiped with long firearms, and there's another one inside de palace in a room with a balcony. Furthermore, there's a fast travel point on the roof of the palace, so once we have killed the five guards, we can fast travel again and they'll be respawned.
Jamiereloaded23 stated that with the weakest pistol we can shoot the guards more than once before killing them, so we'll manage to get more countershots before we need to fast travel to get the guards respawned. Thanks to him.Edit2:
As BombasticGinger said, "if you are too far away, the game doesn't seem to register countershots. If you walk forward until your target gets outlined in white, then countershots will work/happen (you don't lose ammo when it works)