Okay after looking around both Cu and Achilles feats and proffiles, if you look at the wiki for Achilles here it straigh up says"Cu Chulainn is capable of fighting Achilles to a draw"which is impressive on both of their ends, Considering Achilles has no way of evoiding Gae blog as even having EX ranked Agilty would be useless, unless they get out of range before it's activted, Achilles has managed to counte fighting with his spiritual core crushed, Both have insaie endurcae, Cu having 12 hour long fight with Gilgamesh and regardless if Gil was taking it seriously or not it's still highly impresive as he lasted longer that Heracles did when he fought Gil. Achilles is stated to be a top servant on the level of Karna who's stated on par with Gilgamesh. This is one of the most even fights so far that really could go either way with very high difficulty.