Fate/Swapped Chronicles (Chronicles of Darkness; Mage the Awakening + FGO, in that order!).

[Repentant_Dragos]

After reading the PDF (again) and getting some advice, I was told that Prime 5 alone cannot dissolve Gilgamesh, as Gilgamesh, being a Heroic Spirit, qualifies as a Rank 5 Ephemeral Being for the purposes of being attacked with CofD-style Magic. Not merely that, but even if I add Spirit 4, it would still require Ritual Casting to get around Magic Resistance.

But thankfully, there is a solution, one that allows Sandy Blond Kid to be subdued and captured later on to serve CHALDEA - Give him Time 2 or 3, which allows him to cast a ritual spell before he is Isekai'd into Gilgamesh's summoning room, then 'freeze' it in time to unleash on Gilgamesh once he is transported to Novum CHALDEA.

However, he only has one such spell prepared, and so he either has to bluff that he can do this regularly (he cannot), or he gets jumped and captured.

And depending on the Ritual Spell, he does not need extra ranks in Spirit either; he just has to cast Aetheric Winds or Celestial Fire (spells that harm either physical objects or dissolve Mana) ritually beforehand, 'hang/freeze' the spell with Time 2, and then cast it in panic once Isekai'd only to find out that it was his only spell capable of instantly harming a 5-Star Servant and he just used it.

This, in turn, proves the validity of Mage the Awakening's Magic System if used right without making the Nasuverse's system in any way useless.