An even easier method is to attack 'friendly' non-hostile enemies, i.e. Redcoats.
Go to Fort Soleil in western River Valley (at coordinate 122, -551 on the map) after you've taken it over (you'll need to be at least past Sequence 3 Memory 2 to do so). A group of nine Redcoats will be standing in tight formation in a courtyard, with a convenient platform on the wall above them. You can easily line up a shot that hits six of them.
This method also works for:
Edit: Sneaky tip from x Mataeus x for earning both achevements quickly: "If you use the sleep grenade first, you can berzerk them all after they're sleeping :D"