[@Hinsek][@RainDash]
Oh, I didn't know that about his myth.
Well, we could follow the nasuverse version of his ability (didn't know he even existed there tbh) and make him immune to all damage besides divine damage until he gets hit in his heel, permanently disabling his noble phantasm?

I'll keep the shield-wielding in mind during writing.
As for the summoning process... Maybe she assassinated somebody who was about to summon a servant, and she continued the summoning out of curiosity, just to see what happens?
It could work imo.