This trophy is when you first meet with Takuma (The rifleman on the rooftop).
You see 2 guards static conversing and 1 guard running back and forth and static conversing. Fortunately, there is a crane supporting a plank above two guards, so once 1 guard is running to the two guards and starts conversing, open fire on the plank and it'll topple on 3 guards and you'll win the trophy.