The easiest way is to set aim mode to Casual. That way, all your shots are locked on to the head.
Another easy way is to travel to Jorge's Gap just west of Armadillo. Throw a bottle of Zombait into the ditch and the already dense mass of zombies will converge on the spot. Using the Rollingblock rifle I was able to get 2-3 headshots per bullet fired and unlocked the final rank with ease.