After gaining the side mission Enigma Conundrum and netting
You need to proceed to the courthouse where a fight will ensue and you'll save a hostage, this is not the one intended for the achievement. Afterwards you'll receive your first radio frequency for the Enigma Box riddle which will unlock the location of the first hostage to save for the achievement in question.
Follow the marker for the Riddler Hostage side quest now and you'll find a ? symbol painted on the wall, *also a political assualt you can stop if you haven't yet*. press "A" on it and Batman will knock on the wall and open the path to the door. Once inside the first door turn around and there'll be a ? with a switch to hit with a batarang to open the door to the main room.
**If you've unlocked the line launcher you can skip over all the panels and Riddler makes a crack about you cheating**
Inside the main room the floor consists of electrified panels connected to the lit up ? marks on the walls, hit the corresponding ? with batarangs and progess along the safe path before time runs out to the next dead panel. Advance this way until you make it to the lift then glide to the platform with the hostage to free them. The hostage will give you another frequency with the next riddle for the Enigma box and your next hostage location (provided you have enough Riddler trophies found)