Unlike One Piece, this manga is more realistic in terms of limits and potential. No matter how much progress Clare made, it was always clear that she just didn't have the potential to fullfill her ultimate goals by herself. Even her achievements so far required the help and even the flesh of others, like it was the case with her arm.
This simply is not a manga of trying hard and surpassing everyone with that work.
This has nothing to do with what type of series it is, this has to do with good story-telling. The entire THRUST of this series was Clare's goal of killing Priscilla to avenge Theresa. That's what we as the readers were promised, that's what it has been building up to. And for the author to go, "Eh, it's too hard to realistically develop Clare to the point of being able to beat her, I'll just have Theresa do it instead" is just bad, lazy writing. Like if you were incapable of doing that realistically, don't make Priscilla so uber elite in the first place.
Also, there's a WORLD of difference between Clare powering up through the flesh of other warriors and ANOTHER warrior doing the fighting. Hell, it'd have been better if Theresa's flesh somehow awakened and gave her Theresa's godlike power, it would have been a lame asspull but at least it would have been Clare fighting!
I figure Teresa is going to do the grunt work psychological damage, get Priscilla down to a more "mortal" form… but it'll still come down to Clare (or Raki) actually finishing Priscilla off. Teresa is just soloing it far too super effortlessly above and beyond the abilities of everyone else at this point for this to really be all that there is in the final showdown... if this is just to whittle down a power that's been built up too highly, I'm okay with it. If this is actually the end... then its just not satisfying and the entire series really has been a waste of time.
On the other hand, this is basically the level of overkill the anime used to rush their ending seven years ago, so... shrugs Maybe Claymore really was always going to end with a single overkill powerup transformation.
Even that I'm bugged with. Again, imagine if Shanks mortally wounded Blackbeard and all Luffy had to do was finish off a weakened opponent? It's just not satisfying. Granted Priscilla is uber megamondo broken but it just rubs me the wrong way that it's her mentor who she's trying to avenge that's doing the heavy work, like if her comrades did it, it'd be one thing but meh…