This achievement is unmissable. You will unlock it for rescuing the 2nd Riddler hostage (as the description says
To unlock the hostage's location you need to unlock a certain percentage of Riddler's trophies and/or solve his riddles. The location of the hostage can be found next to the church (the building to its right). You will notice a green question mark on a wall. Push it to enter the building.
Solving this "riddle" is actually very easy. It is a riddler-fied version of shell game. Simple turn your Bat-vision on by pressing
and follow the guards way. tell Riddler the correct position of the guard and the hostage is saved.
(video credit: Thebiohazardgames)
Hope this helps you guys!
Any improvements? Please let me know in the comments below!