Under the Easy difficulty setting, get an auto-shotgun and find a Witch. Quickly approach the Witch with lights off and force her to rise, then rapdily shoot her head as she stands.
Get an assault rifle and charge a power shot (start with pistols, then hold LT + Y + RT at the same time). If done correctly, your character will hold out his/her hand as if to grab an object. If you hit a Witch under the Easy difficulty with the charged power shot, she will die. You will get the "Cr0wned" achievement regardless of whether or not you hit her head.