Because the assassins are difficult to see due to their camouflage and high 
speed running, try using slow-motion with L2. You will notice them immediately 
once they are slowed. After that, let all of your rounds into them. If you do 
not have slow-motion (for example, in the bonus mission when playing as the 
demolition man), you must have a quick finger and a good eye to notice them 
running around. There are two ways to do this. The first is to toss a grenade to 
reveal the assassin for a certain amount of time. Hopefully the splash radius 
will either hit him or kill him. Note: The grenade has a two to four second 
delay before exploding, and correct timing is required. The second way is to 
wait until the assassin approaches directly in front of you. When he reveals 
himself, start firing at him.