Run at enemies and hit and hold the melee button, sometimes you'll attack the enemies, sometimes you'll do a finisher. If you only hit the enemy, you'll want to press the button again, and you'll likely get the finish.
It's really easy to miss this trophy but if you only try to use your sky hook you'll get it very early.
Remember you need to HOLD the melee button, otherwise you'll only be doing a quick hit.