Cu should be put as equal to, if not above, Scathach, since, despite his own downplay of his abilities vs hers, he has canonically killed her at least twice without madness enhancement (once in myth, once in Extella)
Artoria should be put into a higher tier than Cu because Artoria lmao-rofl-stomped him despite having AGI 2 ranks lower than his. She even defeated his casaulity reverting NP, which he was put into a corner and forced to use.
In fact, I'd honestly put Gawain and Artoria in the same tier, or Artoria just barely below him. I went and rechecked the fight between her and lancer and she was definitively more skilled than him, but held herself back in their second duel. Even then, Lancer was hard pressed and could not find a solid advantage, even against a handicapped Artoria. Additionally, we have varying info on Artoria and Mordred's duel, but from the scenes in Fate/Apocrypha are anything to go by, Mordred didn't stand a chance against her and was utterly destroyed, with the fatal wound to Artoria coming from Mordred's corpse and Morgan's curse rather than her actual skill.
It's also been stated that a "wielder of the Sword of Promised Victory" was what defeated Atilla, so there's also that to go by.
So all in all, Cu Chulainn and Scathach go into B tier, Mordred either stays or gets promoted to B+ tier, Artoria gets promoted to B++ tier, and
Tl;Dr: Don't underestimate the face of type-moon