Well, do you have any other ideas? I used to work on this on 1.10.2 until I had school stuff and came back to this a long while later. In 1.10.2, you could have used .getHeldItemMainhand().getItem() and work from there. However, I guess in 1.12.2 they changed it so you can't use that anymore. Btw, .getHeldEquipment() returns an Iterable<ItemStack> to my knowledge, the code I put up there was just as far as I gotten to.
Edit: IceDice9, grabbing the information from the player would work since I can use .getHeldItemMainhand() from there, but I need to grab it from event to make sure I am getting the source of the damage, not just a random player (I don't even know how that would go on a multiplayer server)