In order to fight Akuma, you need to defeat the main characters in the Tekken 7 story mode. After that, playing as d. Kazuya, you can fight Akuma in Tekken 7 special chapter. If you win, you will receive the Master of Iron Fist achievement.
Akuma's main danger is in powerful strikes at close range, as well as fireballs with which he attacks from a distance. If you want to win at any cost, just change the difficulty level of the game before fighting with him to "easy".
If you want to show your high skill in the game - do not change the difficulty level. So, Tekken 7 how to beat Akuma manual below.
To defeat Akuma, you need to reduce his huge health reserve with ranged attacks. D.Blaster (right kick + left kick) is suitable for this. Use Reverse d.Blaster at mid-range. Until the very end, try to keep him at a distance because of his dangerous punches. It is impossible to defeat Akuma exclusively at a long distance, and at the very end, he will get closer and begin to strike with his hands.
If you block the first two hits of his normal combo, you get a chance to start your d.Fist combo. To do this, press (forward, forward, right punch), and once it connects keep tapping the right punch button to continue the combo.
Important! If you are unable to make a quick combo at close range, then try to retreat to a long distance. The main thing is not to let him block your three attacks in a row.
If you reduce Akuma's health bar to the minimum, he will use his most powerful, ultimate ability (Akuma rage art). When he triggers it, you'll need to simultaneously perform your Rage Art by tapping R1 on PlayStation or RB on Xbox One / PC. If you do it right, you will win in a Kazuya vs Akuma Tekken 7 special match.
P.S. I still think, that after a lore's appearance Akuma Tekken 7 story (or Tekken 8?) is just a matter of time. And maybe in the future, we will receive Akuma vs Kazuya battle, where we will play for Akuma. Also, Tekken 7 story mode chapters are a bit too short, but nonetheless amazing.