After the first Riddler puzzle in the church you'll have to make your way to the courthouse. Save the hostage there and you will get the location of the first hostage that is connected to this achievement.
Make your way to said location and inspect the wall with the huge green question mark. Inside you'll have to solve a pretty easy riddle involving some batarangs to discharge the floor panels. Always go from one safe haven to another. As soon as you reached one of the panels that aren't charged look for the next question mark to aim at. Continue to do so untill you have reached the elevator. aim at the last question mark to lift the elevator upwards.
Alternatively, you could just wait until you have the line luncher to just skip the whole playing ground
As soon as you'll untie the hostage, the achievement unlocks!
(video credit: Fliptriq)
Hope this helps you guys!
Any improvements? Please let me know in the comments below !