Done via Random Style Change mechanic.
Usually, after a battle 1 point is added to the Styles counter to determine your next style. If the point should be added to the currently Maxed Level Style you are in, all styles get 1 point instead. Meaning once 100 battles is hit for a Style Change, the game will randomly select a Style because they all have the same amount of points. I had to get hit in each battle as the Guts Style counter wasn’t enough to stop the Shield Style point from being added.
During the time the next Style is being selected:
If the most points are in the Style you are already in, or if it’s Shadow Style in White or Ground Style in Blue, those Style points are reduced to 0.
In the case v1.0 of EXE3, this does not apply. This allows the opposite version to obtain the other version’s specific style as demonstrated in this video.