The "Someone" is the "mysterious [hooded] figure watching" you in the
Watcher in the Wings side mission (available after your initial visit to the Courthouse).
Simply hit him with a Remote Batarang [LT>+RT]. If done correctly the achievement will unlock immediately after he catches it.
Here is a map of his four locations. Note:
This achievement is missable. If you get too close to him you will trigger a cut-scene and miss the chance. You'll have four opportunities throughout the game; the first being immediately after the Courthouse, but your best bet is to just
avoid landing on the roof he is standing on. Easily done by accident if you are unaware of his presence/locals. If you have already completed this side mission, it
can also be acquired in NG+ mode (unlocked by completing the main story) or alternatively you could start a new game (it doesn't take long to get back to the Courthouse, about 15 minutes).
Video provided courtesy of Wozamil. Warning: the audio does contain some light plot line spoilers.