this MU seems like it's dependent on a lot of factors. for one, are we assuming shunguang is starting off in base and not immediately dipping into her qingming sword form?
base to base, megumi is stronger from the looks of it (6 tons vs. >0.334 tons), but shunguang's durability scales way past what he can dish out so nothing he throws at her is gonna do anything. even with shikigamis, the AP vs. durability gap from the looks of it seems way too wide for megumi to reasonably harm shunguang at all.
if shunguang goes into her white-haired form then she has the potential to just one shot megumi with any of her attacks. obviously the big deciding factor here is whether or not megumi manages to summon big raga in time, which if he does, i think raga would just steamroll shunguang, not only scaling higher but shunguang herself also not really having any way to keep him down permanently
would shunguang killing megumi before raga kills her count as a W for her? if not then i guess maybe i'd say incon? megumi himself can't really do anything to hurt shunguang but shunguang can't do anything to hurt big raga