You can code that manually quite easily. Create a few variables and when the player presses the attack button, make the engine ask if the enemy is resistant, meaning that if some variables like skeleton_resist_slash == True. Depending on the answer, you can edit the damage values manually. If you do this in large scale, you should have the exact system that you need in your game. I think we need more general features in RPG in a box since people make vastly different games, not just rpgs. Specific damage types would be too specific.
