Choose Ectomorph body and Wing Chun fighting style, then dump stats into damage, speed and optionally dodge and you'll win within 50 years for sure (my fastest is 20). Boost damage first because there's virtually no chance of winning within 10 years, so you'll want to lose fast on purpose beforehand to get back to upgrading (you'll lose from having virtually no speed). Once damage gets to 600 or 900, then boost speed to start winning, then add dodge to beat high speed foes (and make getting health/armor useless).
At least that's what I did. Now all I don't have are the Luffy and Naruto medals.
I say this was a fun way to spend a few hours; a bit of humor, a bit of experimentation.
For those wondering what the stats do (wish there was an in-game description):
Speed: the faster fighter attacks, then their speed goes down*.
Health: (pretty obvious what this does)
Armor: The more you have, the less damage you take. It goes down as you get hit.
Damage: (also obvious)
Dodge: percent chance of not taking damage from enemy attacks. High dodge upgrades slowly.
*Speed goes down depending on how much speed a fighter has: lots of speed means it goes down much more quickly. However, as long as you have more speed than the enemy, they won't attack. Also, once a fighter's speed reaches 0, it goes back to what it was at the start of the fight.