Encounter: Evasion (Chapter 7)
Select the Evasion Encounter from the main menu. The first enemy in this encounter can be pulled while hanging from a ledge (press square while you're on the ledge under him).
Restart the checkpoint 20 times and repeat.
Be aware that your kill count resets each time you start an encounter. So if you quit the encounter you need to do 20 kills again. You must use checkpoint restart 20 times in a row.