Fairly easy to complete immediately after receiving the pepper grinder. You will head out back of the woman's house and there will be bolterflies spawning from nests in the area. Just kill 5 in a row and achievement unlocked.
UPDATE courtesy of GnomishNinja : The one thing that I'd add to this is that the Focus ability really helps. Hold LT and you will automatically focus on the next opponent. If you keep it held down, it will automatically direct you to the next enemy. You can use RS to switch enemies, if needed.