The theater is a quite a big location where there are six enemies armed with guns. I recommend that you eliminate the enemies one by one, without haste, while remaining in shadows for the whole time and selecting the ones that walk alone. I absolutely recommend against taking the risk of direct confrontations because, not only will you gain less experience, but also it will be more difficult to keep Batman alive. Still, if confrontation takes place anyways, use smoke pellets and hit fire extinguishers with batarangs in an attempt to lose the enemies. To get around the area, it is best to make use of the statues under the ceiling and reach them using the hook.
You can use the abovementioned statues to your advantage, of course, while eliminating successive gangsters. A good idea is to perform reverse takedowns, which you can perform right after the enemy walks under the protagonist. Right after you perform this kind of attack, remember to get away from the site as quickly as possible (the best idea is to get to a statue located at the opposite end of the theater) because the other enemies will quite quickly appear in this area, and they may notice the lurking Batman.
Another method of using the statues to clear this location, is to glide towards those of the enemies, above whose heads there appears the bat symbol. Once again, I remind you of picking the enemies who walk alone. After you hit the enemy, you need to finish him off and leave the site as quickly as possible. to prevent being spotted.
You do not need to restrict yourself solely to planning attacks from above. An attention-worthy idea is using the passages under the floor, which you can reach by raising the interactive grates. You can also use this place not only for hiding, but also to attack by surprise those of the enemies who happen to stand in the vicinity. You should remember, however, that you should not stay in the region after you eliminate a given enemy.
There also are several weakened walls in the theater, which you can explode using the explosive gel. Using this gadget is especially recommendable if you have upgraded it with Proximity Detonator, thanks to which you will not have to monitor the situation personally and wait for the right moment to detonate charges.
Continue taking the steps described above which will ultimately result in clearing the theater of all six bandits. Before you leave this place, you may talk to a Falcone thug, although it is not necessary. Go South to reach Penguin's office located in the Boiler Deck, which was inaccessible earlier. This will result in a cutscene being played, featuring Candy and Alberto Falcone.
[New Character Profile: Alberto Falcone]
[New Character Trophy: Alberto Falcone]
You now need to fight another battle, which is a bit shorter but, this is not to say that you can ignore the enemies. You should mind the enemy with a knife and, due to the small size of Penguin's office, do not let yourself be cornered . After you win the battle, you will see a cutscene and you will get to fight a battle with the next boss, which is Deathstroke.