I don't really get that you think that Rufus is the weakest from Sabertooth. I mean, his magic is limitless. His Memory Make can copy any magic that he wants. He can also create new magic with spells he memorizes. What if he memorizes Minerva's spells? And what if he creates a new magic based on her spells?
Rufus' magic is limited by his magical power. Even if he can copy other people's spells, that doesn't mean he can match their magic power. Minerva's strongest spell involves summoning one of the Yakuma War Gods to perform an attack for her. I don't think Rufus can create an entire war-god from scratch. That's some Zeref-level "pulling life forms from the ether of magic" stuff. Rufus could try to summon the one Minerva summoned, but he would have to beat Minerva to the chase. That's assuming he can in the first place.
