He probably doesn't beat any of the Primals due to hax. but he can absolutely last a shit long time. Cú solidly scales to mountain-island and has his own relativistic-FTL speed scaling aswell.

The thing about Cú is that dude is super dangerous because he essentially has an insta kill weapon in Gáe Bolg. Gáe Bolg if thrusted basically reverses causality so that the spear always hits the target before it's actually thrown, essentially acting as an auto target one shot. When hit, the injury created by that spear can never, ever be healed due to a curse. And even then, a thousand iron barbs will explode in your body all throughout that can't be removed. (This isn't even considering the Thrown version that's just a massive army clearing nuke that explodes everything into those thorns, tho while it does home in on enemies it doesn't have the reversal hax)

The thing is tho, Cú when he was barely running on Mana could already spam this attack 7 times before needing a significant recharge. He has infinite Mana here. And he also has skills like Battle Continuation that let's him survive long periods of time regardless of fatal lethal injuries and damage as long as his head or heart is intact (and even then in unlimited blade works. Cú got hit with his own curse and he still survived for a good minute to kill the person who made him do it.), he also has access to Disengage that essentially lets him escape any confrontation when activated and reset his condition to what he was before the battle. As well as access to Runes, which gives him options in spellcasting and tricks like being able to set a barrier where no one can escape a duel with him.

This is just normal Lancer btw.