Take out the Vincenetti gang.
You first start fighting off the goons in front of Police Headquarters. To fire Carmelita's shock pistol, press . Click to enter or exit strafe mode. Strafing is recommended, as it will allow for more accurate shots.
After taking out the goons in this area, head over to the large bridge to take out a few more. Then, a few more will appear near Octavio's opera house. After clearing those thugs out, return to the police station for one last round, then the job is complete!