Well first of all this ain't a "Code Problems" post in my oppinion.

Second, this is a question thats WAY to general.
Its like asking people to make a game for you.
In most games the battle system is everything uc.

Now this is maybe a answer you dont want to get but the only solution is to check some demo's.
Type in "Combat" in your search bar and i bet you will find anything of use.

Letting out ac is all up to you.
If you want to put it in your combat system oke but remember what the use is of each stat.

Example:
Make the damage depend of the str and def and then go look if you hit something (agi and acc) and then inflict the damage. It really not that hard to make.

Oh and btw, i would really recomment a turn based battle system. Those systems are good in lagg and will make the combat more fun cuz you have to think what to do (use a skill or defend or what ever) instead of just hitting the attack button.

Lots of succes, greetz Fint