Ah, so it’s that sort of boost skill.
In Manga and Anime, quite a few secret skills put large burdens on the body, and make it so that you can’t fight again after you use them.
But when the main character faces an extreme dire situation, he uses it and loses his ability to fight. He either miraculously recovers, or awakens to some other power.
But it’s not as if the world is that convenient for us.
It’s likely that Rishia’s ability to take in Chi is on another level.
The amount of Chi she had from the beginning didn’t get that high even when she raised her level. That’s the only reason I can think of.
And no matter how long Firo and Atlas can keep Mussou Kassei up, there’s still a limit.
Using only internal Chi, it’s one minute. While taking Chi in, five.
But Rishia was able to use it throughout her battle.