In Turn Your Back To The Sunrise(Mikagami Hiyori), Reimu confirms she would all-out without hesitation kill Marisa if she were ever to become a Youkai(Hence turning her back to the Sunrise/Marisa).
This means Marisa would have beaten or at least double-knockouted a Unrestrained Reimu(Including Fantasy Nature) in order to still be around for the next generation. No wonder she’s a Grand Youkai despite being only like a century old, she probably got the title by throwing down with a Reimu trying to murder her.
(It’s not like she’s *that* important in the grand scheme of things for someone powerful enough to interfere and save her for breaking the laws of Gensokyo).
At least learn to fly indeed, Marisa could’ve annihilated the New Shrine Maiden in a single spellcard.
Edit: Well assuming this has the same canon as Mikagami’s other works.
Updated
WHY, YOU—
*fwyuu~*
GNNH...!
Yeah, yeah. See you around.
A 100-something grand youkai, and you're going to run!?
This is your final warning!!
*SNAP!*
*FWYUU*
At least learn to fly before you talk so big, little miss maiden.