MR negates a spells existence via vague and unknown means. This makes it very subject to NLF rules, so anything outside of what it's interacted with tends to get hit by a bat.
No? We are told precisely how Magic Resistance works and even see how it operates in many different fashions:
Magic Resistance (対魔力, Tai-Maryoku?, localized as "Anti-Magic"EX) grants protection against magical effects. Differing from the Resistance effect that merely rejects Magical Energy, this ability cancels the spells altogether.
Here, Artoria's armor is created from her own magical energy. Here, Artoria's armor constructed from her own magical energy is called an "Impregnable defense". Here, a discussion about Bellerophon, Artoria notes her magic resistance/armor nullifies all but Sorcery or Fantastic Races with Divinity (after all, Divine Mysteries are nullified by stronger ones).
Here, there is only 10 meters between Medea and Artoria at the start of the battle. Here, Artoria starts sprinting at Medea, Medea reacts and casts a magic that increases the air density around Artoria enough to halt her forward momentum. Here, Medea used Great Magic with only one word, such a feat isn't possible for magus like Tohsaka without her gems. Here, Artoria's magic resistance dispels Medea's magic.
Magic Resistance all but breaks down the spell and the Magical Energy itself that makes the spell on sheer contact, only passing through if the person allows it and honestly, Magic Resistance itself isn't needed as shown with stuff like just having enough Magical Energy is enough to break through Curses and stuff like Medusa's Cybele makes it harder to just say that MR is "ill defined".
People agreed with this take and this is the only part I will talk about as everything else is just a mess of contradictions and stupidity that relies on this and nothing of how Ainz or anyone in Nazarick can hurt Alcides.