This achievement is fairly straightforward. Defeat the first boss of the game, Misuzu. The boss can be found at the school's entrance, the final room before leaving the school.
The most effective tactic for Misuzu is to attack with your three hit combo (X, X, X). Mizusu will then be stunned. Attack twice with your heavy attack (Y, Y), and run away (Double tap Left or Right. lternatively, double tap Up or Down to travel across the vertical plane). Mizusu will perform a power attack on wake up.
As the fight progresses, Misuzu will make more frequent dive bomb attacks from the air. Run across the level to dodge this attack, keeping an eye on her shadow which will indicate where she will land. Continue to land combos and ground attacks until Misuzu is defeated.