The ending kinda breaks the willing suspension of disbelief.
I get what you were aiming for, but you got the wrong girl for the job.
Her track record makes her the canonically weakest member of the GDF, as she's the only one among the GDF members to have canonically lost to the boss from the stage before her own.
You should've gone for one of the onis.
So does Kasen, and she's a powerful oni hermit. She's way above Meiling in every way.
And the oni species in general is known for having physical strength beyond other youkai species.
Meiling, meanwhile, couldn't beat a lone ice fairy. She has NEVER defeated a single named character in canon. She lost ALL her onscreen fights. She knows martial arts, but those martial arts couldn't win against the magic of an ice fairy. So there's NO WAY she could possibly win against an Extra Boss.
