Fast-travel again. The police precinct is located in the Burnley district, i.e. in the Southern part of the city. If you have not sabotaged the communications tower there, yet you can fast-travel to the Coventry district and go to the South-East. Regardless of your route, approach the precinct from the North and, after you listen to the conversation with Alfred set out towards the rooftop. It is here that you will bump into seven armored policemen and, since they are guarding the only possible entrance, you will have to eliminate them all. I recommend that you plan on all the steps that you take from now on, while remaining near the gargoyles.
For obvious reasons, I recommend against engaging the corrupt policemen into fight because, since they are armed, Batman could die very quickly. It is much better an option to attack them from behind and escape onto the gargoyles, or into any other safe location, after you perform each attack. Start by eliminating the sniper occupying the balcony shown in the above screenshot.
The roof's main part is being patrolled by three policemen. Each time, wait until the character that you are targeting walks away from the others and strike only then. If you do not want to alert the enemy to your presence, do not attack from above and land somewhere behind them. After you sneak up to them, perform a quiet takedown.
As the next one for takedown, you should select the enemy patrolling the helipad located to the right of the balcony with the sniper that you have just knocked down. What is interesting, you do not need to worry that the attack will be noticed by the two other guards standing next to the door. Also in this case, I recommend that you perform a quiet takedown only after you have reached the helipad, instead of performing an air attack.
Assuming that you have done everything according to the plan, there are only two more guards left to eliminate, who are guarding the building's entrance. The situation may evolve here in two different ways. If you have noticed one of the policemen going left, wait for him to stop somewhere near the ladder, land onto him and eliminate him.
If the guards are, at all times, staying near the building's entrance, then you will have to use one of the gadgets in your inventory. You can encourage the enemies to go out on a patrol by hitting one of them with batarang, or knock down both of them, so you can then run up to them and finish them personally. In the latter case, I recommend that you use the either the concussion detonator or the hook (to pull the enemies towards yourself). Regardless of the variant, end by taking the door to the room in the precinct's attic [Rooftop Maintenance].
[New Character Trophy: SWAT Officer]