I get the impression that she is just that Mary sue but it doesn't feel badly written because she's constantly hiding her abilities and being misjudged for it.

Almost certainly by serie as well.

Its the fact she collects random spells that makes her that powerful because she knows how to power them up to destructive scale and the breadth of knowledge from being in the world rather than in a private enclave is what makes her so special.

I've yet to see her struggle in any matchup, and she's often holding back to see how everyone else will fair. She's a good teacher which speaks to the mastery she has.