It took a while of research, but I've found the real way of unlocking Akuma after unlocking Sakura w/ Ryu, Dan w/ Sakura, Fei Long w/ Abel, Gen w/ Chun Li, Rose w/ M. Bison, and Cammy w/ C. Viper:
1. Set your Arcade settings to 1 round, 99sec, and minimum Difficulty LV of Easy. Also, might be a good idea to turn requests off so that others won't disterb the process.
2. All you need to do is get 1 Perfect and not use any continues. That's it. After you beat Seth, a slash should appear over the screen signaling Akuma's appearance. If nothing happened after Seth, then you must not have met the standards.
3. Beat him quickly. He is a heck of alot of trouble to fight, but, lucky, his defenses doesn't match his strength. Zangief will have a party with him.
4. After he concedes defeat, he becomes playable.
Now that Akuma is in, train with him for a bit. You'll need him to help unlock Gouken, Ryu and Ken's old master.
1. Beat the game as Akuma. You can set it Easiest if your having trouble.
2. Set the Arcade settings the same as before with Akuma. Requests, must definitely off. You'll need to focus for this one.
3. Choose someone you know well. Now here are your stipulations:
1 Perfect 3 Ultra Combo Finishes(not Super) 5 First Hits And No Continues
Yeah, alot of work, but trust me it works. When your finish with the stipulations, your free to fight how you want. You should keep a count of what you've done, so you won't get confused.
4. Same with Akuma. After Seth, a slash should appear over the screen. Your fighting at the same place as Akuma, but this time it's Gouken, stronger in both offense and defense, master of the RyuKen style. He will Not be as easy as Akuma to take down, but I still recommend Zangief to end the battle quickly. Master how to get to get in close to the opponent and get his throws out quickly.
5. He becomes playable afterwards.
Now that he's in, train. For he's the key to unlocking Seth if you've already Beaten The Game With Every Other Character.
1. Set the Arcade setting to Easiest, 1 round and 99sec, to get it done quickly. When your done, Seth will be unlocked.
Note: Seth, in reality, is weak in both offense and defense. To not take much damage, you Must train to learn when to pull off moves that will save you from being KOed quickly.