Generally, for finding the starting numbers: you start with two out of three numbers, duration (of encounter), damage per turn, and total HP (= total damage dealt in duration). To set them in relation is some basic math, like so.
Example: an encounter takes 10 attacks (turns). Each attack deals 10 damage, thus the enemies in the encounter need to have 100 HP (damage x attacks).
Example: you want that enemies have 1000 HP, and the fight should last 5 turns, then you need 200 damage per turn.
Once you have some sensible basic numbers, you can complicate it. For instance, 1 STRength = 10 damage. So you derive some basic stats. Or you factor in 20% of attacks are misses. An encounter actually has 3 enemies, thus you divide the total HP between them and so on.
The numbers should be sound in an calculation sheet, and you can then see that you simulate how it plays out. This should be entirely numerical at first, to see that the predictions are right.
I would prototype visuals in a different scene, and hook it up with quick key down actions. This allows you to quickly test how the attacks etc look like and that they work. Finally, you put both together.
Start with the reduced complexity. E.g. no item use yet, only auto attacks and one example special attack etc. Then gradually bring in new features.
