I'm building a game called DVP(Digimon Virtual Pet), and in this game other than taking care of your digimon, You can also battle and breed them. I'm working on the battle system (making it first cause the actual pet system will be easy compared to the netplay, or 39DLL)but here is the problem I don't want it to be "too" simple or "too" complicated, but I do want to go by a certain formula. There are str, def, spd, and int.

• Strength: How hard the attacking digimon is hitting
  
• Defense: How much damage your digimon can defend when being attacked
  
• Speed: The chance of you missing the enemy
  
• Intelligence (battle knowledge): The chance of you hitting a critical hit or defending a critical hit.

I can make a super simple turn based example, but I don't know how exactly to make the formulas for what I've explained above, any help?