You simply need to play ranked matches online and win a whole match (3 rounds) with every character on the roster. This includes Akuma. Keep a checklist of all the characters you have won a match with. As you win, make note of who you used so you don't repeat victories as a character. This is the fastest possible method of earning this achievement.
To fight as Akuma online: Go to the middle of the top row while selecting your character highlight either of the middle two characters and press up. (From the starting point, Ryu, just press right, then up.) Akuma's portrait should be displayed. The portrait looks like the silhouette of a dark, ominous figure.