This is a secret achievement called That's No Hero - Defeat the Hero in Scenario Campaign Mode. It is a story achievement and cannot be missed.
When you get to the G Corporation areas, a level appears called Millennium Tower Heliport. It starts with a fight against Anna and plenty of low-level henchmen. Once these are defeated, Kazuya will appear, jumping out of a helicopter. Defeat him to complete the level and earn this achievement.