Just finished getting Calla. Probably logged a dozen or so runs of her final stage, which - theoretically - should have a 25% chance of getting all three medals (assuming you run 3 slower teams on the bottom path and your topmost path team can solo the optional boss). Never got that gem.

On the other hand, I got to watch Sarcandra nearly solo the final boss, because she evaded almost every attack. She just refused to go down. That battle took like five minutes.

RNG is especially temperamental today, it seems.