There are multiple guides on Youtube for both normal and expert. In practice some grinding in the first area is helpful and some fights are RNG heavy, but the majority of the game is actually really well balanced for it, especially since you get Pictos/Luminas and weapons specifically designed for that playstyle.

Personally, I killed Simon on my second try by rebuilding my party to intentionally die to his attacks, proc some key Luminas and trigger the start of a high damage combo. The game rewards creative builds and lateral thinking.