As it is, I feel monster damage types are balanced, but not fully. The way their types of special attack, weaknesses, and resistances are determined is odd in my opinion. Out of all the monsters, Constructs and Beasts are the only ones who are weak to the attack type they use, and resist the other’s attack type.
I propose these be switched such that:
-Beasts inflict and resist Piercing damage, and become weak to Elemental damage, and that
-Constructs inflict and resist Elemental damage, and become weak to Piercing damage.
This would change the balance in the importance of knight’s weapons, as currently Elemental is seen as more all-purpose, and Beasts’ attacks are generally easy to avoid. Also I feel it would make more sense for a monster to resist its own attack type.
If still not convinced, take a gander at the damage type resistances of gear crafted and awarded from boss tokens (ex: Barbarous Thorn Shield).
I know I won't convince you but lopsided things are beautiful too. From scratch (since this only works from scratch anyway- we're not about to change the game fundamentally) how would you design the damage types? Are damage types necessary? What do they accomplish?