This is the first mission in the riddler side-quest ark.
When I was about to save Mr. Freeze from the Penguin I received a notification, telling me to go to the church. After I arrived there I got a small speech from the Riddler who told me to solve his first riddle.
To crack this nut you simply have to scan the organ insinde the church. The achievement will unlock right away.
(video credit: Napalm Donuts)
Hope this helps you guys!
Any improvements? Please let me know in the comments below!