This achievement can easily be done in the very first chapter of the game “Prelude”. While going through the level, you go through a tutorial of the game. At one point it will tell you to press the button to go into focus mode and to use to do a vertical slash on the enemy. You need to watch the shining light on the blade when you start a focus attack. Hold the button down and wait until the light on the sword goes to the very tip of the sword. When it does, release the button to perform a perfect vertical slash and cut your enemy in half. Doing this will always cut your opponent in to as opposed to just dealing heavy damage to them (except for female ninjas, samurai, and other heavy weapon enemies).