You meet the first charging rabbit on level 3 (The Forbidden Mine). All you have to do is roll (press ) out of the way to dodge it's charge. After you have dodged it's charge, melee it for a few seconds until it stops being dizzy and then dodge it's charge again and repeat. You have to kill it by doing melee attacks, shooting it will void this achievement. If you get hurt by it or shoot it by accident purposely kill yourself to retry it. There is more than one charging rabbit so don't despair if you mess up on your first try.