This is a story-related trophy that cannot be missed via normal campaign progression.
You are given a "list" of names. The men on this list are scattered across Tuscany and must be killed. You can go in any order you like, it does not matter.
The final target whom you need to assassinate is
For this to happen, you must engage in a very slow moving "follow" sequence. Take cover and do not be seen by the target or his goons following him. You must make it to the Templar meeting before you can kill him. You can also hire Courtesans or Thieves to help keep you hidden from the eyes of evil. When you arrive at the meeting you will get a small cutscene and then be prompted to kill your target. Avoid all other soldiers and sprint for your target and kill him quickly. Then flee the area to complete Sequence 5.