I've been playing Pokemon Shuffle for a while and I've gotten up to to Expert Stage 19 (Yveltal) and I've gotten an S rank on many of the main stages up to 190.
I know there are more Expert Stages to unlock, but what are the requirements?
I've been playing Pokemon Shuffle for a while and I've gotten up to to Expert Stage 19 (Yveltal) and I've gotten an S rank on many of the main stages up to 190.
I know there are more Expert Stages to unlock, but what are the requirements?
Expert stages are unlocked based on the number of main stages the player has achieved S-rank on.
As of now (version 1.2.6) there are 24 Expert Stages, those you are missing are Mewtwo, Genesect, Chesnaught, Delphox and Greninja.
Here below you can see which Expert Levels you haven't unlocked yet.

As the developers are still adding content and so new Expert Stages, I suggest you to check Bulbapedia after each update.