The Hands of Fate
Oh and if you're curious: The 100 Medal prize is Robust Lingerie, which gives +23 Defense, +100 Style and a 1/16 chance for enemies to be stunned for one turn (which stacks with the similar effect from the Luminary class, but is still too miniscule to be of any practical use).
And if you want to see the alternate ending you get if you defeat the optional boss within 20 turns (which requires copious amounts of grinding):
That's at least a pretty good payoff. The even bigger bad swears fealty to you and then trashes the final boss without breaking a sweat. Imagine beating Emerald Weapon in FF7 and then he goes and kills Sephiroth for you.