Hunting the Penguin

Head to the Ferry Terminal and go to the rooftop to meet with Nightwing. You'll also receive the Disruptor. Shoot the weapons crate and sabotage the ammo of the two thugs below. After doing so, go down and knock them all out.

Next, fire the Disruptor's tracker ammo on the van and knock on back to spook the guys. Now you can go to the rooftops and follow the vehicle. Continue stalking them until they lead you to Penguin's base. Once there, clear the roof of enemies and sentry guns. Head inside and find a hatch on the floor that you can go through. You'll find Penguin there and his thugs.

Find a vent on either side and slide down under the floor. Get behind Penguin and grab him. Counter any attempts of his goons to attack you from behind. After the Interrogation, Nightwing will appear to your rescue. Beat all the thugs out with his help.

Once the room is cleared, enter the vault and place some explosive gel on the explosives then shut the vault door. Detonate the blast gel to complete the objective. This will also unlock the Gunrunner missions.

Leave the hideout from the roof only to find more goons waiting outside with sentries deployed. Stay up in the metal beams then take control of your Batmobile. Conveniently, there's a parking building in front of the hideout. Drive your vehicle to the top floor to have a clear shot at the militia. Destroy the sentries and take out all militia soldiers.