There are two places where you can get this achievement.
The first place which is only available once is at the beginning of sequence 8 memory 5. Here you will use Leonardo's Flying machine. Directly in front of you are several guards on a rooftop.
to lock a guard nearby then fly low enough and you will kick him automatically. If the achievement does not unlock crash with the flying machine and try again or play the mission to the end and use the second opportunity.
The second place is a special memory after you reach sequence 12. The mission is located in the north of the Forli map on a tower with a viewpoint (marked on your map with an white assassins symbol).
You can repeat this mission as often as necessary. So if you have missed the achievement in sequence 8 this is the only place in the entire game where you can get the achievement.