When you go to Jerusalem and do the assassination where the guards are surrounding you, do not try to fight them off. Instead, turn around and run up the ladder to reach the second floor. A short intermission sequence will show your target running away and up another ladder. Hold R2 + X and run across the chandeliers, then run up the ladder to the right. 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 L2 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 to 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 finish the memory.