This trophy is attained on
Episode 1 - Back to School. Once on the rooftop, the game will introduce the scanner to you for the first time. After "Tag" walks into the building, scan the 2 enemies near the front door, 3 enemies on the couch on the first story, and the 4 enemies on the second floor.
Now wait for the story to progress, and two white vans will arrive. As they are coming into the parking lot below you, make sure to use your scanner and tag all 8-9 enemies inside the vans, which is when the achievement/trophy will unlock.