@N4mm1nh 100% agreed. I love Kyoukai as a character, though.
I will say this: I try to think of it metaphorically. The way the Big Boys fight (Moubu, Shin, Ouki, etc.) is every bit as unbelievable as Kyoukai. The weapons they swing through multiple humans reminds me of Dynasty Warriors video games where you can kill 1,000 enemies in 20 minutes.
The metaphor, though, is that this inhuman skill is just their strength as leaders: The energy they give to their own soldiers and the fear they cause in their enemies. Imagine the poor farmer who has to square up against a fully-armored Moubu on horseback. Moubu is going to seem 10 feet tall and bulletproof. Kingdom makes Moubu look 10 feet tall and bulletproof so we can "see" what the lowly soldiers see.
And for Kyoukai and this whole "dancing assassin who can bring back the dead" is a metaphor for the spiritual beliefs the people of this manga have. She seems magical because everyone believes she is.
At least, that's how I reconcile the more absurd parts of this manga. And just for the record: I'd never think this deeply with any other manga, but Kingdom is one of the best ever written, so it's worth the effort.