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 RB + A 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 LB to lock 
onto your target while still chasing him down. When you get close enough, the 
word "Assassinate" will appear for X 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 
complete the memory.