This achievement is unlockable from Chapter 11 onwards and it requires the time of day to be night. If playing in Premium Adventure mode you can use the door next to the bar in Serena (where you save, change outfits etc) to change the time of day.
Collect Haruka from the Serena bar and take her to the West end of Park Blvd (North of the map, below West Park). You should see a crowd of people gathered and a gentleman looking to jump off the roof of the building in front of them. A cutscene will follow. After this, enter the building using the door in front of the crowd. Another short cutscene will follow and you now have to talk to the gentleman from the roof.
Use the following statements to successfully complete the "Man On a Ledge" substory and grab your achievement.
"Tell me what happened."
"Who is Mizuki?"
"Have you contacted her?"
"How much have you spent on her?"