Successfully complete Story mode to unlock the "Instant Carnage" special chapter, which involves Kazuya fighting Akuma in a final match. You can select this match in Story mode, after clearing all story chapters. It is a difficult fight, even on lowest difficulty. Akuma will block most of your attacks, teleport a lot, and shoot at you from afar. One easy strategy is to repeatedly keep pressing LB + A (with an Xbox controller). This move gets you close to Akuma and lets you jump over projectiles. By repeatedly performing this move, you can defeat Akuma approximately 50% of the time on lowest difficulty. It does not work every time, but it does not require any skill either. After draining Akuma's health bar, keep pressing RB (with an Xbox controller) very quickly to activate your Rage Art, otherwise Akuma will instantly kill you. Complete this special chapter to get the "Master Of Iron Fist" achievement.