There are lots of stories of situations like this like with David and Goliath or the Horatti, but they all seem to be legend. How would a fight like this work and would the losing side accept defeat or continue fighting against their word?
There Was An Attempt back in Greece. u/Iphikrates explains why it didn't quite work out and the attending problems.
As always, more can be said as to similar 'champion' stories elsewhere, so if you can contribute an answer of your own, please don't let this post stop you!