Effectively a really stringent Monomove+Monomon. Take a Greninja with the ability Protean, and teach it any type of Hidden Power. No other moves and Pokémon are allowed. Win 5 before losing 5! Check the Common Rules for the rest of the rules.
Since this would be too hard, challengers are permitted to switch the Hidden Powers between matches.
Also, don't try this. You'll end up losing horribly. But then, if you win, you'll be the third one in this Hall of Fame!