"Just" chi?! Chi is more than just vitale energy, it's the PRIMORDIAL energy, the primary substance that composes everything in the universe, including time, space, life, motion, existence and essence. So it's no wonder Kai wins.
Bowser: there will always be someone to stop you… Kai: who? Mario??? He’s chi is strong but it won’t be enough,.. he will meet the same fate as you… *chi’s bowser*