It seems that Monica really is somebody capable of dealing with Aoko somewhat. Of course, her magecraft is inferior to Aoko's own and far inferior to the Fifth, but the very fact that she dealt with a genuine Dead Apostle in less than a minute is truly indicative of her skill. This woman is incredibly deadly. In fact, you'd say that Monica is likely even better at killing than Kiritsugu was. Well, at least potentially. The Magus Killer's reputation for killing isn't exactly undeserved, and even if Monica is potentially superior to him thanks to her apparently incredible skill in magecraft, Kiritsugu's true strength was never really his actual physical ability to deal with threats. You doubt that even Monica as she is now would be able to survive standing on an anti-personnel mine she was unaware of.

Still, her magic would be pretty fantastic if you ever found yourself with Kiritsugu as your enemy, not only is it horrifyingly powerful by itself is is also a form of magecraft that so easily counters the use of weaponry like firearms and potentially even the Black Keys of the Church. Though you don't know just how the magical ability to manipulate magnetism or metal like Monica was apparently showing off would extend to magical constructs like Black Keys.

it's really quite funny. If Kiritsugu is the Magus Killer, then Monica has practically the perfect power-set for being the Magus Killer Killer. Good thing for Kiritsugu that he decided to retire, otherwise he may have found himself put up against his natural counter. Y'know, an actual natural counter, and not somebody like Kirei who just sort of ran at a guy with a gun and almost killed him. In fact, if Monica wanted to, she could likely kill Kiritsugu with ease unless the man took specific precautions against her. All it would take is for him to be carrying a single hand grenade and that would be the end of him.

Part 2