GCPD Assault

Head back to GCPD and talk to Oracle. After that, exit to the garage and you'll find two militia APCs coming right in. You can actually enter your Batmobile and easily take out the enemies easily by running them over or shooting them with the non-lethal rounds. Otherwise, you can just give them the old fashioned beating.

Once cleared, use the Batmobile and use the winch to remove the ventilation fan. Grapple there and make your way out. You'll see a large enemy force surrounding the building. Head to the generator outside but you'll discover that it is destroyed. Make your way to the garage door entrance and squeeze inside. Do it fast or the drones outside will detect and attack you.

On the road leading to the garage, you'll find more militia along the way. Defeat them then power up the generators. Now you can call in your Batmobile and engage the drones outside.

Oracle will be able to help by hijacking some of the drones as well. Shoot the targets that will be highlighted to trigger explosions and EMP blasts. Later on, Cobra drones will join the battle. Prioritize them and Mamba missile launchers to give you much needed breathing space.

After destroying all drones, Scarecrow's militia will drop foot soldiers on the rooftop. Head there and engage them to battle. You can use Oracle-assisted environment takedowns but it's better if you'll be able to use them against hardened targets like Brutes. After defeating the first wave, another wave of militia soldiers will arrive, including two brutes equipped with different weapons. Use Oracle's assisted takedowns as well and take note that your REC can give you much needed space by stunning enemies for a few seconds.

After getting rid of the threats in GCPD, you have to go to the movie studio next. Also, all the remaining explosive devices and outposts will become available in all islands in the city.