Dear lord. While I ended up having more trouble with The Fly than this fight, at least I understood conceptually how it was possible to beat The Fly hitless. This fight, on the other hand, felt so intense and brutal. Maybe my favorite fight of the route, bravo on pulling it off.
You get it by playing the Athenian Rhapsody Demo, which grants you a token for completing it. Redeeming the token when starting a playthrough of Athenian Rhapsody causes Thunder Goober to appear at several places throughout your run, giving you a gift based on what you did in the demo. The Corn Outfit is one possible outfit, and I know there's a Wizard Outfit as well.