You need to find specific pressure pads scattered on the rooftops of the buildings to start this task. You have to use the remote batarang to scan the rioters and find Riddler's victim, which will be highlighted red. Once identified, you have to use your REC and hit the target to disable the explosive implanted on his brain. It's easier to shoot the target when you haven't scared them so stay in the rooftop, approach the victim silently and shoot them from a distance.