So. New chapter revelations. Apparently Pricilla is so powerful because of the overwhelming hate-on she has for youma.
Actually, it sounds kind of like the source of Neku's power in The World Ends With You. The reason he was able to use all the different Psychs was because without memory, his imagination could run wild.
Pricilla, without knowing why she hated youma, only that she hated them, could "hate without limit," giving her so much power.
Conversely, someone like, say, Ophelia, could only hate until she understood her brother's sacrifice, at which point she was willing to give up her life.
Clare hates Pricilla, but because she's only quarter youma isn't as powerful.
I get the feeling that with this explanation Clare would have been a very high ranking warrior if she had been fully half-youma.
Eh, just some speculation.