This is derived information:
A Duelist "deals X-Y damage to enemies with rigid skin", using the pickaxe.
There is no "rigid" property in the game, and the check for rigid skin is if the foe are either inorganic, or among a small set of anthopods. ( Source ).
A Wraith has properties of being both Undead and Inorganic, and thus would practically be considered having "rigid skin" when attacked by a Duelist's Pickaxe ability. Conclusion: Wraiths have rigid skin.
A Mimic is instead considered Demonic, so even if it might appear rigid, it's not.
