Whatever misgivings I have in the direction Claymore has gone in since Pieta, I still believe that when it does eventually end, that Yagi will be ending it as a story of redemption. If "defeating" Prisc is part of the end game for the manga, than I don't really think she's going to be killed off. If defeating Prisc is just a stepping stone to Yagi coming up with an even more Aizen-like Aizen, then sure, I'm sure Claire will pull another proverbial rabbit out of her butt and power up some more.
