Hate to bust your bubble but I actually did this twice...first time was with Garchomp, second time was with Mega Gengar. They were ~Level 68 so not terribly overleveled either.
Although to be fair one is Garchomp and the other one is an Uber...
Overall I felt that Diantha would have been cool with more character development, shame she wasn't really involved in the story. Her team also isn't horrendously tough aside from Goodra and Gardevoir (and Tyrantrum I guess if it gets a Head Smash off).
Not horrendously tough, agreed, though varied enough at least to me. I guess what I used just didn't have the same effect, because I didn't really wreck her nearly as easily as other people seemed to. In retrospect using Goodra and Clawitzer may not have given me the best view here.
Totally agreed on the character development, no question about it. She could have been better if they did more with her. Maybe I like her because of her potential more than what actually happened... Oh well, you make some good points here, and now I should probably play through XY again so I can see what you all were talking about with being able to sweep her team.