Another challenge based one, obviously.
Kill 6 grunts with one whip snap.
You'll probably not get this until late in the game. I tried on Act 2 - Federal Archives, but it was too tricky, grunts where just streaming for all sides, hard to get enough in one place. I unlocked it on Act 2 - Port Anderson I. After the first 'Saving...' point, just climb a lump of debris on the right, near a bus and let them come towards you, there's easily 7 or 8 at once there.