You can use Akira to defeat even the highest ranked players just by spamming Y,Y move or going back far from your opponent and when they come to you use forward, forward , B, B move (double jumping kick which opponents don't usually block and keep repeating it) By doing this method I was able to defeat even Conquerors when I was 10th or 9th kyu. And also like Button Masher mentioned star will appear when you defeated this perticular opponent so look for opponents that don't have star next to their names.You can also check your arcade percentage completion and see how many opponents you have and have to defeat.Just go to home menu then status and view rival list.