The statement is about her turning the "entire space" into ash, and comes from Prillya.
Thing is, the space refers to the Mirror World, which we see later turns smaller and smaller the more Servants they fight. Heracles is fought in the equivalent of a single building. I posted a scan as well that shows the walls of the Mirror World look to be far too close to the river and the bridge despite the fact there's a whole ass city to either side of it, so it that shouldn't scale to the full size of Fuyuki.
Edit: Shadow snipped me.
I already gave my whole thought process about 6-C earlier. There's Gawain surviving the blast, there's Karna taking Balmung to the face, there's Arash surviving Arondight Overlord but pretty wounded, etc.
Of course, Karna reduces attacks to 10% of their power, and Arash has A rank Endurance, multiple statements about a super resilient body, and an EX rank Skill which does multiple things like reduce the damage he takes. But it does consistently show that despite NP being generally stronger than Servant's base power, it is not by literal hundreds of times.
And we already have a big number of Tier 6 stuff for Noble Phantasms (Attila destroying Rome, Bennu being destroyed, potentially Spartacus matching and completely destroying a meteorite so thoroughly, there weren't even big debris left to fall on the village below, Ishtar nuking Uruk and making a big hole, Gugalunna being able to destroy the entirety of Connacht, which would scale to Ishtar's second NP which summons Gugalunna's hoove to come raining down, Ozymandias' Dendera Light Bulb being compared to Solar Flares, Santa Quetz's NP being compared to the meteorite that ended the Dinosaurs).
So either NPs of similar rank have an AP difference of dozens or hundreds of times, or they have a similar level of power and Servants can survive some of these with powerful defensive abilties that don't boost their Endurance by dozens or hundreds of times.
Again, Low 7-B, Possibly 6-C feels like the best option because Low 7-B already has like 2-3 calcs supporting it, but 6-C also has one and is internally consistent with the power between Servants and NPs. Hopefully that makes my opinion more clear.
Edit: King Hassan is a Grand Servant, so don't use him as an example. Beasts, and Grands by consequence, are far above Normal Servants and shouldn't be used for normal Servant scaling.