Potenxia, as a mage, is very proud of her skills. She is also *extremely* self centered. Those two factors combined and are part of the reason why she has decided Cassie was her rival due to her ability to deal with spirits, something Pontexia cannot do.
The fact Cassie also makes minor potions means that Potenxia also got angry on her girlfriend's behalf, and her warped view of crimes means that she feels as though Cassie gets no consequences for her actions, while she gets punished. this rivalry is woefully one-sided.
The best way to picture it is if team rocket, instead of harassing a plucky anime protagonist, went after an indifferent middle aged woman who could not care less about them out of pettiness. Her goblin gf just goes along because she has fun with it either way
