Get up close and in front of the enemy and keep spamming that melee button.
If that not works, take some distance where you won't do a takedown and keep spamming that melee button and than move towards the enemy.
What also might work. Go to a liberated outpost, wait for the Rakyat warriors to show up in their car (your allies). Go stand in front of the car. Hold
to commandeer their car. When they get out, start meleeing them. If you provoke them, they'll will count as enemies. Not sure if it works with the skill requirements, but you can get multiple trophies by doing provoking the Rakyat which mention enemies in the trophy description.