The Warriors

The Flashback chapters are backstory missions that unlock after completing certain amounts of main story missions. In order to unlock them all, you will need to complete up to the 11th story mission "Boys In Blue". You can complete them at any time as they unlock & they each have their own high score & secondary objectives. There are five flashback missions in total & completing the last one will net you this trophy

I haven't tested if you can just unlock them all and then only play the last one for this trophy, but you'll need to complete them all for the 100% completion anyway. This Trophy Popped for me after completing the 4th one.