If you start at the ground floor, you will have to find stairs #1. Otherwise you just need to follow the only possible corridor #2. Approach the windows and watch a cutscene with Two-Face.
Turn right and use the ladder found here #1. After reaching the upper balcony, press and hold the right trigger, carefully approach the nearby sniper and press Y for a surprise attack (you can also finish him off by pressing X) #2.
Now follow the instructions received from the game and go along the wire #1. Stop above the bandits, aim at one of them #2 and execute the already well-known surprise attack (X).
Some of the enemies will run away from this room, though you will still have to take care of those who stay #1. Just like in the previous fights, try to perform counters #2, use various types of attacks and try to keep a high multiplier.
Right after taking care of them, Two-Face will shoot at you #1. Luckily you won't have to fight him, as Catwoman will do it for you. The further part of the cutscene will show Joker's failed sniper assassination attempt #2. Batman will decide to determine where the shot was fired from.