Quincy Sharp's current whereabouts is east of the subway terminal exit. After getting there you should see the mayor being bullied by a group of inmates #1, so it would be good to plan an attack from above #2, using one of the nearby buildings to your advantage.
Note that the enemies are equipped with firearms, so you have to be very efficient at attacking and finishing them off #1 and use Smoke Pellet when things get ugly #2. Others gadgets might also come in handy here, for example the Disruptor upgrade allowing you to jam the enemies' firearms.
Keep fighting until you get rid of all the thugs. Stay down, find the mayor #1 and press Y. Batman will interrogate him in a quite unconventional way #2 which will bring new light onto the accomplishments and further plans of Hugo Strange. Now you can resume the task of reaching the GCPD building.