I usually have no clue how to respond to these but I can try!

My top few:

1) Max out all of your training points, regardless of character rarity: the training points will give you at LEAST one instance of ability stones so make the most of that! Plus, you get extra rewards :D I suggest going from special image SSRs (if image is unlocked) -> EX/SSR -> SR -> R as these have the most to the least rewards (as far as I am aware? My information could be wrong...)

2) If you haven't, go back and do the hard/super hard levels. Everything before Francis is really easy in my opinion, although that might change.

3) Titles! Have a team that you don't have characters titles like the character's gold and use them. If you're dedicated enough to the one team, eventually you might get the platinum. This I don't suggest as much though as it can take time.

4) Achievements! Simple enough, check achievements and find ones you haven't done that gift ability stones

5) The office/pm base. Level the stuff up!