This is one of the most difficult opponents. It's not for nothing that the Demon King entrusted him with one of his important artifacts.
To defeat such opponents, you need observation and strategy: for example, you already know at what point he starts cheating. So, at this moment or a little earlier, you need to distract him somehow. For example, like you did with the monster in the mushroom world. Or you can use Celica's charm... Or you can find other ways...
Yes, this boss is difficult, but he also has weak points.
You can read about the game mechanics in the Agranor library.