Prepare for a bit of grind for this one and its best to try and do it as you progress to avoid back tracking. For this trophy you need 100% sync / complete all optional objectives in the following:
All 9 main story missions
All 8 Divinci dissappearance mission
All 6 romulus missions
All 4 war machine missions
6 templar agent missions
12 assassination missions
10 thieve guild missions
10 courtisan assignments
None are missable as the can be replayed from the DNA menu. To trigger the thief and courtisan missions you need to visit the guilds for these two factions and revisit them to unlock more.
Your current total will show on the DNA menu.
I would suggest getting the crossbow, smoke bombs and a full assassins guild as soon as you can to help with the objectives.
I used the following two resources to help:
https://www.trueachievements.com/a147995/il-principe-achieve...http://uk.ign.com/wikis/assassins-creed-brotherhood/Walkthro...Also if your getting stuck on a long mission then upload your save to usb stick or the cloud when you pass a safe checkpoint so you dont have to do the whole mission again if your getting stuck at the same point.