I am calling bullshit on her not having some sort of supernatural enhancement, like... when she arrived in this world her strength was comparatively scaled by the standard of this world.
Her fits of strength and endurance can not be justified when you consider that one bullet in her head guard made her faint, it simply doesn't make sense.
I know this is a silly story and I won't get overly hung up on that, but if the author is pulling "she is strong because she is a samurai" shit only I will be at least annoyed.