Arkham Knight

Grapple to the next ledge to find another Riddler Trophy across the ledge. Just take note of its location for now then continue forth until you reach Gordon's location. Position yourself under the grate so you can perform an optimal Fear Multi-takedown. Eliminate the other guards to attempt and Gordon to trigger a cutscene.

The Arkham Knight has taken position on a vantage point so you can't grapple out of his view. You have to carefully sneak towards him and take him by surprise.

After catching him, he'll change positions and will call in reinforcements. You have to take them out one by one or by Fear Multi-takedown if needed. There's a riddler trophy under the middle table in the bar area as well.

The Arkham Knight will be controlling the drone so you have to sabotage it with your Disruptor. After eliminating the guards, get near the Arkham Knight using the grates on the floor. There's also another free Riddler Trophy located directly under where he is positioned. Grapple on the vantage point where he is at to lower his HP. After that, he'll lock you up. You need to escape the enclosed area or you'll get smothered with gas. There's an opening on the gate itself. Use it to escape.

In the third phase of this battle, you have to face another wave of his militia guards. These guards have optic deflection armor, meaning your Detective Mode won't be able to see them and you have to rely on your eyes. Once done, sneak up to Arkham Knight once again to lower his HP.

Finally, you'll have to deal with him, his drone, and his sentry guns. Use your RHD to blind and get behind them, just make sure Arkham Knight's not aiming on you. There's also another Riddler Trophy right under his vantage point as well. This is quite easy compared to the other phases of this battle so you shouldn't have problems sneaking past the turrets and the drone.

After getting him off guard, a cutscene will trigger. Once done, free Gordon. Another cutscene will take place and you have to deal with more militia guards once more. After defeating them, join with Gordon and take the elevator. Watch the next cutscenes.