Small tweek on Dr Gonzo 815's method.
Load up a private game: Private on Extra Content Maps> Training Ground> San Donato
Now, Gonzo is right, the A.I. doesn't like to walk around, what I did differently was use Throwing Knives to hit my target from a hiding spot then follow them, they will move from crowd to crowd as you do this, once they are in a crowd near a bush or haystack, hide in said bush or haystack and hit them with another throwing knife and they will slow walk/stumble right passed your hiding spot giving you your chance. Rinse and repeat.
I also used the perk Overall Cooldown so I didn't have to wait as long to use my Knives again.
This worked for me in under 5 minutes. Hopes it helps.