Once you have activated the assissination follow the icon to the target. Once you have entered the courtyard a cutscene will play.
Once you can move normally again climb the statue in the middle of the courtyard and jump on to one of the ledges that run either side of the statue. Run the length of the ledge towards the balcony where the target was speaking and jump onto the flagpoles. From the flagpoles you can then jump onto the balcony where the mark was giving his speach.
This traget does not have a weapon but he does run away constantly, therfore select your secret blade an run after him when you get near him hold RB and X to perform a quick kill, wathc the cutscene and the achievement will unlock