Look into the "element_rate" method in Game_Actor and Game_Enemy, as well as the "elements_correct" method in Game_Battler, which is used in the "attack_effect", "skill_effect", and "item_effect" methods in the same class.
This will allow you to see how the damage is altered depending on the elements. You can also insert your code to alter it, depending on how you create the equipment to possess varying rates.