Well, before you even start, I suggest figuring out some basic equations for stats. I.E:
DAMAGE = E.ATTACK - DEFENSE * WEAKNESS
This means that when you are attacked, your defense is subtracted from the enemy's strength, then the difference is multiplied by your weakness level. Very simple, but if you have other stuff in there, it can get kinda complicated.
DAMAGE = E.ATTACK - DEFENSE + E.VIGOR + E.MENTALITY - SPIRIT * WEAKNESS
And etc. As more stats come into play, things will quickly become muddled, so I suggest figuring out how you want these things to play into each other.