This is an unmissable trophy, as it is part of the story. These are the steps required, for those who are wondering (minor spoilers):
After the car crashes, break out of the car and walk towards the officer. Grab the keys to the handcuffs and unlock them. After the scene, crawl back to the car, look right and grab the shotgun shell, then look left and grab the shotgun. Fumble with the shell, then make sure to get a head shot. Trophy unlocked!
Here's a video from 4K gaming (player control gets started at 5m00s).