Bionic Commando

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.

The earliest convenient spot is during Act 2 - Ascension City Park II

At the start of this mission you will be outside and must drop into a building. After you exit this building you will come upon a weapon pod and directly across from it is the area where you can gain this achievement. There are more than 6 grunts here so if you need to fill your adrenaline bar kill a Heavy grunt with the Bulldog weapon. Once your adrenaline is full just keep hiding behind the many objects here until 6 grunts get fairly bunched up and when they do jump at them and press Y+B to whip them all and complete your challenge. FYI this area also has the levels 3rd collectible which you will no doubt see while waiting for the grunts to bunch up.