Bully Achievement in Assassin's Creed: Revelations
Find and beat up Duccio.
How to unlock Bully
The character Duccio will be introduced in Sequence 4 Memory 3. He will run off after you confront him. Afterwards, he will remain in the Constantine District. Once you spot him, there will be a red icon on the map. Likely, you will spot him without trying. When doing a Master Assassin Mission called "The Guardian, Part 1" you will be in a market which is very close to his location. If not, just go into that district and go to the Blacksmith shop in the North part. He is behind that building.
Note: This character will not appear in game until you have completed Sequence 4 Memory 3.
Here's a video for further assistance: