Iv ARK ANGEL vI's soulution is decent but he's missing one of the hallways so I put together one in a little more depth.
320 Riddler challenges are required before the location of the fifth hostage room is available. It's in Amusement Mile across the bridge from the Gotham Casinos. On the roof behind the watertower you'll find the ? mark on the wall to press "A" on.
Once inside the main room head right and slide under the projector get to the end of the hallway and see the next obstacle you'll need to slide under currently blocked by a crusher. Aim above it and fire a shot with RT from your REC to pull the crusher up. Time your slide to go under it, the next crusher behind it and finally another slide to go under the low gate to make it through.
Next up is a hallway with four pads, a bunch of hanging ? marks and an electrified path in front of you blocking your progression. Stand on a pad and take aim, individually at the lit up marks, don't quick fire batarangs here. Each time you hit one the countdown timer will start again. Hit the required amount of ? before the timer runs out to proceed, hit the wrong ? mark or run the timer down and you get a shock.
Enter the next hallway and hang a left up a ramp, three steel, stacked crates are inbetween a breakable wall and an electrified barrier, turn around and fire an RT REC shot at the generator to pull the crates back then fire an RB REC shot to send them through the breakable wall. Now you need to guide two remote batarang shots up and over the electrified barrier at the ? mark switches to shut down the barrier, proceed down the hall and up the ladder.
Line launcher time, send a line across the gap of electrified floor pads, change direction midway to your right down the grinder tunnel and hack the Riddler panel at the end, it's a lvl. 2 hack and has a few incorect answers, the correct one is "Bamboozling". Line launch back through the grinder and head right. Grapple up to the next lvl.
You face a corridor of electrified pads look up towards the end of the room where the projector screen is and grapple up, turn around and you'll see two generators on the ground, fire a RB REC shots at them to lift the crates in the cage above them and they'll crash through the breakble floor below. Glide back to the starting point, look up and fire a RT REC shot at the genny to get the crates in position, then look down at the original two gennys on the floor, on an angle and fire a RB REC to send the crates through the breakable wall at the end. Grapple back towards the end of the hallway and proceed to the final corridor.
It's cake from here if you've got your line launching skills down. Launch a line down the hall and get ready to change direction to you left at the end, continue down this next hall and change direction to your left again midway this leads your to the hostage platform and a sweet, sweet 40g