This trophy is story-related and cannot be missed via normal campaign progression.
You will be doing this sequence during the famous Carnivale in Venice. You will need to win 4 mini-events so you can win a special golden mask. Upon completing these events, you will still have to steal the golden mask.
After you get the mask you must enter the party. Your target (Marco) is on a boat that is slightly docked just outside of the city perimeter. Use your hidden gun and blast him.