This is a story-related trophy and cannot be missed via normal campaign progression.
In this sequence you must stop the assassination attempt on the Medici family. After you do that, you must find your target and kill him:
All this will take place in Florence, the city you started in. Remember to climb up the view points and open up your map some more. Note that you will fight the target once on the ground but will only make him retreat. You will have a cutscene with Lorenzo (Medici) and then be sent to find the person responsible for the attack. Your target will eventually be on a rooftop that you must climb to be able to assassinate him. There is a small chase once you reach him but it's simple enough.