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.