Hanger Trophy in Assassin's Creed III: Liberation
Hang an enemy on the gibbet at the Place d'Armes
How to unlock Hanger
The Place d'Armes is the area where your first riot (with Blanc) takes place. It is also located in your Location DNA in the main menu if you want a visual image of the place. Just go there, place yourself on the gibbet and hang an enemy with the whip.
Credit to abdullahkankuro for the video.