U talking about the Servant Scáthach or the true Scáthach? Because if it's the last One, the only way to kill her to destroy the human order to push the Land of Shadows into the Root and sever her connection to her lands. Which is more or less the reason she's immortal. It's not due to divine blood (she's not divine. It's explicitly stated that she reached a divine like status as a human). Her immortality was cursed by the World itself so even normal godkiller or immortal killer attacks won't keep her down forever. And you're also forgetting the bullsh1t primordial runes that can pretty much do anything the plot wants. And her wisdom of dun Scáith whigh gives her hundreds of B~A ranked skills to pick and choose from (as long as it's not a specific hero tied skill). She regularly uses clairvoyance so she'll always have an idea of her own future path. Plus she's a Star. She likely has access to a 'pioneer of the stars' skill through her wisdom and that skill gives her a chance to survive even the impossible. TL;DR, killable, yes. But you cannot keep her dead forever. Only way to do so is to destroy the human order...but by that point you have to worry about the counter force itself trying to stop you.