@Lazy
The Nasuverse actually has a ranking system for this, but normally this is tied to the number of verses required for the spell's chant, which obviously is rarely employed in all works.
We generally go off AP since it's a good basis for the amount of punishment said Magic Resistance is able to take. For instance, Saber's A-Rank Magic Resistance allows her to brush off High Thamaturgy like Medea's beam spam. However, she herself stated that "True Magic", or things that are not able to be replicated through the science of the Nasuverse, like Time Travel, would bypass it completely. Meanwhile, Archer's D-Rank Magic Resistance would stop simple spells like basic curses, but he'd get utterly obliterated if he took a direct hit from Medea's beam spam.
Nanasen should bypass Saber's Magic Resistance since it's attacking with a physical medium, with the magic aspect being the splitting of the wires. However, I don't think simple spells like illusions, mind manipulation, and curses made with it would work all that well, given that Medea can't use these things against Saber. I also don't recall Kanzaki having a particularly powerful nuke spell or a super-strong attack outside of Yuisen.
As for mana, I don't remember the Empyrean Eye being particularly mana intensive. Mystic Eyes as a whole aren't that exhaustive except in certain cases (like Medea's Cybele, her Mystic Eyes of Petrification). But NotEvenHuman has played through this part of the game that hasn't been released in North America yet, so I'd defer to him for answers about that.