I've not read that far I know she lose in one of the fights that when she learns that tanya killed her father and she gets obsessed with revenge. I remember one of her allies commenting on it.I don't know if this is true but I heard later in the light novel she messes up and gets someone killed. I don't know about later but assumed tanya's side was going to lose so unless she single handedly stops the empire I don't know if I'd call her a mary sue. But Idk it's hard for me to call a antagonist a mary sue because they need to at least for a little while rival the mc. I'm going to reread the novels and see if i missed anything i just got the newest one I think it's volume 6 or 7. Ps were you read the light novel or web? Because from what the wiki said she was purposefully a mary sue in the web but the auther changed her in the light novel to give her more character. Check the wiki link above it says that under trivia.