There will be a crawl space you can enter on the west side of the walkway. You will find this riddler trophy at the Gotham City Police Department underneath the staircase in front of the building near the top left on the north side. Then follow the ramp down to the lower level and use Explosive Gel to destroy the weak wall to get the trophy on the ground. You will need to use your Remote Electrical Charge (REC) gadget to open up the outside door to the GCPD and slide underneath it.
You will find this riddler trophy inside of the lower level parking lot of the Gotham City Police Department in the same area where you found Riddler Trophy #45 but in another closed off area. You will need to glide down from the northwest building and smash through the wood to get the trophy inside. There will be a wooden weak wall on the north side of the parking lot.
You will find this riddler trophy inside of the lower level parking lot of the Gotham City Police Department and east of where you found Riddler Trophy #44. Make sure to use the Batarang boost to hit the switches in time and the breaks to slow it down and gain control. You will have 7 seconds after you hit your first switch to hit the next 2 switches. You will need to step onto the green pressure pad and then use the Remote Controlled Batarang to hit the riddler question mark switches across the street in the caged-in area. You will find this riddler trophy underneath the street leading to the Gotham City Police Department and east of Riddler Trophy #42. Then glide from that pad to the south lower green pressure pad for the cage to open. You will need to glide from the southwest building onto the north green pressure pad. There will be 3 green pressure pads on top of 3 buildings in around this trophy in the cage. You will find this riddler trophy southwest of where you found Riddler Trophy #42. You will need to time your run and slide correctly in order to get the trophy as you slide underneath. It will be inside of a long cage at the middle of it rotating up and down. You will find this riddler trophy on the rooftop of Gotham Casinos. It will be inside of the shed, use Explosive Gel to destroy the wooden weak wall to get it. You will find this riddler trophy on the rooftop of Gotham Casinos to the northwest of the pool. You will find a green pressure pad to the left side hallway of the patio then look in the opposite direction to the north and fire your Line Launcher to smash through the wooden walls to grab the trophy on top of a barrel at the end. You will find this riddler trophy at the entrance of Gotham Casinos. You just need to glide down from a high point on the GCPD building to fly into the green pressure pad and then immediately use your grapple upwards after the hatch above opens. It will be inside of a cage on top of a platform that in order to be opened you need to hit the green pressure pad on the wall below. You should be able to just jump over the fence this way.You will find this riddler trophy in front on the building to the northwest of the Gotham City Police Department on the east side of it. Here's a video, but don't watch past 30 seconds or so, if you don't want to know what's behind the door :) This guy jumps down and glides over the fence, but I think you can also get enough height by firing the line launcher parallel to the fence, then standing on the line. I believe there was a trophy behind this electrified fence, so you've probably been in this general area if you've found all the trophies already. You'll need to go to the question mark, and then descend below the street level into a tunnel, cross over an electrified fence, then make a left hand turn to find an unmarked door - this is where the Riddler is hiding. (I believe this is near the back entrance to the Iceberg Lounge, by the way.) If you don't have this question mark/arrow icon it's not yet time to find the Riddler - you may be missing some challenges or trophies.
If it's time to take the Riddler down, you'll have a blue question mark with an arrow pointing away from it on your map, in the southwestern corner.