This achievement can be unlocked by playing the stage Aku's Mines on Jack difficulty. Save a Kiai Attack for the boss fight against The Scotsman at the end. Allow yourself to be attacked until your health drops to less than 10%. Sounds should be muffled and you should hear Jack's heartbeat as an indicator of low health. Once your health is sufficiently low, approach The Scotsman and unleash a Kiai Attack with Samurai Fist equipped. This should defeat him instantly, and the achievement will unlock on the end-of-stage screen.