Just curious, why magic from the old world? Why can it not be: Just like the Code of the Lifemaker, the monsters are protected by some curse that renders attacks from those whose origins lie in Mundus Magicus?
Only exceptions being descendants of the Royal Family & people with logic defying strength (i.e. Rakan)
Fake Asuna is from Mundus Magicus. She can use Asuna's artifact, but if she or anyone from Mundus Magicus wields it, then it can do no harm to the "Shadows".
And her cover is gonna get blown soon. She said it herself that it only needed to last till the current day (in Negima).
Her inability to resist Magic has caught the attention of Kurt, Negi & herself, but they all dismissed it as an accident.
Asuna's power is not an actual part of her physiology, but rather some kind of magical power present inside her (Called The Graveyard of Magic if I remember what the Senator told Arika). The Cosmo Entelecheia are after it. It is probably something that only occurs in the Imperial Bloodline as it explains why the Ostian Castle is called the Gravekeeper's Palace. Which would make Asuna or the Ostian Royalties who are responsible for protecting her, the Gravekeepers.
Also, if Fate is an Mammoth, then Negi is a Lion. His(Negi's) chances of winning, depend on special circumstances, like support from Asuna or his Magea Erbea. Relatively, a fully serious Rakan would be a T-Rex.