Kirby does this in a lot of games. The main game is beatable by a child, but the more extra stuff you do, the harder it gets. And the secret bosses like Morpho Knight and Chaos Elfilis feel like straight out of DarkSouls or the Sans fight in Undertale.

Oh, and in order to fight them, you need to have finished everything else that the game offers and defeat all the games bosses beforehand - with limited healing options.