Try looking up a build guide. There are a few online. Getting your CPU team (For the Main story it will be Lin, Elma and Irina 90% of the time unless you want to fill the Irina role as healer/buffer yourself but thats one thing the AI does really well and the battles are too fast paced to keep up at times.) All working together and allowing the player character to be high DPS is the way to go.
I think I am reaching my breaking point with this game. On chapter 5 and combat is just starting to feel like a real slog. I think the battle system isn't so well designed. Like they want it to be streamlined and yet everything behind the scenes is convoluted with all the different variables. In the heat of battle it's hard to tell how optimally you are performing.
I know enemies have different resistances but the game does a poor job helping you keep track of that. Let alone letting you know if your attacks are infective. The thought of needing to go in the menu to swap weapons between enemies types makes me to drop the game.
Duelist and Astral Crusader are the two best player classes by far. In this way the AI will automatically heal Shield and Debuff you. ELMA will have high DPS and Infinite Overdrie and you can just focus on positioning and DPS
I can post a few links if you want.
Also. Id suggest Rushing to chapter 9 before doing any more non manditory sidequests. This game becomes a huge Drag when you have put in lots of hours but are still virtually powerless. At least then you will be around level 40 and able to handle most enemies on foot. From there you can grind to level 60 in under an hour and then ease through most sidequests mechanically while leveling up afinity with previously unused party members