I know I will be stuck on this one. I want to implement a basic, yet enjoyable, RPG system. How can I code how much damage can be done to an enemy NPC or to a character? I want to know how I can code reactions towards elements too (fire>earth, water>fire, earth>water). 

I want to make it simple but fun to play, since game jams are quite short.

there are a lot of tutorials online that can probably walk you through that kind of thing.  are you using Unity?  maybe this one will help?