When you first reach Damascus and must save the citizen, kill two of the 
three
guards. Back yourself into a stall or a corner that you cannot be pushed out 
of,
then hold RT to guard. In ten minutes you will get the achievement. You can 
also
put a rubber band around your controller and RT, and allow the game to idle, 
but
I wouldn't trust it. The games AI can sometimes find ways around it, 
wouldn't
want to get killed only to find out 8 minutes later.