Assassin's Creed II

This is story related and can't be missed. Below is a spoiler do not read it if you do not want to know about the DNA sequence.

You will start the sequence by taking your mother and sister to your uncle so that you can be safe from the people after your family. You will be attacked by a large number of Vieri de' Pazzi's soldiers. Don't worry though as your uncle Mario come with his guards and saves you. Once the enemies are killed Mario takes you to his house. There you will learn to fight and buy armor upgrades and supplies. Once you are ready you will need to go with Mario and you learn what your father truly was, an assassin. You will go with Mario to assassinate Vieri de' Pazzi.

To assassinate him first you must follow Mario to the main walls of the city he is in. You must climb the wall and kill the 4 soldiers on the rooftops with your newly acquired throwing knives. Once they are dead jump down and open the gate so Mario and his soldiers can enter the city. Continue through the city until you reach the spot where you see Vieri. Climb up to the top of the tower he is in and kill him and the few guards he has there. Once he is dead go back to Monteriggioni and talk to Mario about the codex pages. Get the 4 codex pages around the city (they are located on your map don't worry.) Once you've collected them the DNA sequence ends and you will get the trophy.