Ac 2 v 44 - Mark and Execute
0Unlocked After: Sequence 6 - Memory 2: Romagna Holiday
You must kill the target without being detected. He's in a small courtyard by the docks at the north end of Romagna. The best way to assassinate him without being detected is to perform an Air Assassination with the Hidden Blade. There are three archers posted around the courtyard, so you'll want to dispatch them first.
Start by climbing up the south side of the building below the restricted area on the Mini-Map. Sneak up behind the archer on the wooden platform and ice him with your Hidden Blade. Then, drop down to the lower walls and kill the two archers patrolling along there. Use Throwing Knives or your Hidden Blade -- just make sure the target doesn't see you.
Once the archers have been taken care of, walk along the roof, taking care not to be spotted by the target, and wait for opportunity to strike. When the target has his back turned and is below you, lock on to him, move to the edge of the roof and hit the X/Square button when the �Assassinate� option appears in the top-right corner of the screen. Be careful not to accidently kill the woman in front of the two guards - https://www.supercheats.com/guides/assassins-creed-ii/