Get some altitude and start heading Northeast of here. You don't have to go far to find Quincy Sharp. This part is a little tough because it's five thugs right around him. If you have the weapon deactivator or the disarming boomerang then this is the perfect opportunity for those. Deactivate two weapons, boomerang a third, drop on a fourth and then attack the fifth and you should pretty quickly have eliminated the guns from the battle. After that, it turns into a regular fight. If you don't have these, drop in on someone and make good use of the smoke pellets. Either way, interrogate Quincy Sharp afterwards.
After a little bit of talking, you'll be headed to the GCPD. Head there but be careful because there are guards with guns on a lot of the rooftops now. It's actually safer in a lot of cases to glide through the streets than over rooftops. Another safe alternative is riding choppers since they don't check each other. Head over to the GCPD (east) and find a perch. There is a group of enemies down below. Drop down on one of the regular enemies to take him out instantly and then you have to use lots of stuns for the armored guy and the two shields. Be sure to evade a lot on this battle and don't worry too much about combat flow, just try and get it down because this isn't a particularly easy battle.
Once you've removed them, just activate the door as normal and the bottom will thaw. Then you can just slide under as you normally would. Enter the front door of the building. You can't really sneak up on Mr. Freeze here so just walk in the front door.
