When you go to Jerusalem and do the assassination with the guards surrounding you, do not try to fight them off. Instead, turn around and run up the ladder to get to the second floor. A short intermission sequence will show your target running away and up another ladder. Hold R1 + X and run across the chandeliers. The ladder will be to the right. Run up the ladder and another short intermission sequence will show your target on top of another building. Chase him down and have your wrist blade enabled (press Up) Then, press L1 to lock onto your target while still chasing him down. When you get close enough, the word "Assassinate" will appear for Square in the upper right of the screen. Press it and you will complete your task. After another intermission sequence, evade the guards by jumping onto building roofs, hiding, etc. Make your way over to the bureau to complete the memory.