An easier way was to simply ask the developer on his blog. Guess what? You can't destroy the helmet. Don't believe me? Machine translate the original text yourself.
戦闘中に兜を破壊できるようになるアプデとか来ないですかね
もしくはON/OFF機能とか
兜外した状態でも戦いたいです
Could you add an option to destroy the helmet in the next update? Or add a switch feature? I'd like to fight her without the helmet on.
バトルシーンでの兜外しはちょっと実装が難しいかもしれません
できるように組んでいないので……
Implementing a "no-helmet" feature in the battle scenes could be difficult. That's why I didn't program it in to begin with...
Source:
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Conclusion: Wishful thinking. Quote straight from the developer proves he would have done it to begin with if it was easy.