Bully

Duccio can be found after you see him in the cutscene with Sofia in DNA Sequence 4 - Memory 3 for the first time. He is located a little bit north of the Romani HQ and will be displayed as a red marker on your map. You have to knock him out with your fists (select fists as a weapon in the weapon wheel). If you kill him the trophy/achievement won't pop, but he will respawn after a while. You can also go back to the above mentioned DNA Sequence via DNA Select in the pause menu.