When you first engage in combat mode against him (after fighting the duplicates of Al 
Mualim) lock on to him by pressing LT Xbox 360 or L2 PlayStation3 . Quickly equip the 
hidden blade before attacking. Wait for a counter and you will defeat him with the 
first blow you hit him. Note: This is after you fight Al Mualim when he makes about 
six duplicates of himself.