I did something very similar to this, made a new element that only affected a certain type of boss, Then made a new weapon for each class that had an elemental attack for this new made up element. But then i realized that because i made this boss immune to everything excecpt this new made up element, my Mages were kinda worthless. So i made new skills for my Mages that i attached to the Mages weapons so they had magical attacks that also affected the boss.