You will get this achievement a short while into the first level (Day 1) of the game.
You start the game with the Inferno Ninpo. You can only cast a Ninpo after you have filled up the Ninpo bar (the orange bar under your white health bar at the top left of the screen). Once your Ninpo bar is full, cast your first Ninpo by pressing Y & B at the same time. Achievement unlocked.
Note: The game will display a message ("Your Ki is full. Press Y + B to cast Ninpo.") at the bottom of the screen telling you to cast your first Ninpo once you have filled the Ki bar up. You can fill the Ninpo bar up faster by killing enemies with Ultimate Techniques, Obliteration Techniques, sustaining longer combos on enemies and performing other special moves such as the Steel on Bone technique. Have a good one.