So if you haven’t played a Kirby game, most of them have a boss rush. The common name for them is The Arena, and you just fight every boss and mid boss from the main game. In between each boss, you can heal or get a new copy ability, which is a random selection between two abilities. You can pick any ability at the start of the boss rush to use also. I think these ones are pretty easy. But most games starting with Kirby Super Star Ultra have a secret one you unlock after beating everything else in the game, which is usually called The True Arena. In this, you have to fight the harder versions of bosses and mid bosses from the hard mode of the game, along with the any other boss not found in the main game and a harder version of the final boss, usually called a “soul” version, like Marx’s Soul, or Star Dream Soul OS. Besides bosses being way harder, one of the two random abilities in between matches is always Sleep, which is totally useless and will just get rid of your current ability, essentially meaning you only have one available ability every rest area if you happen to lose yours. Also, in The Arena, in between battles you have five healing items to completely restore your health. But here, there’s still five food items, but they only restore about a quarter of your health. It might not sound like too little, but trust me when I say you’ll be getting hit at least a little unless you’re a god at Kirby or using a TAS. And neither of those apply to me. But usually, this is the final challenge of the game, so the difficulty makes sense.