Hey,
It was bound to happen eventually: tonight I drew a random Mongrel Ranger when encountering a Mongrel Wizard. What's the difficulty? I picked a third card, but should I have done differently, like since there is no written difficulty on the card, it should have been 0 and an easy win?
No, keep drawing. Nothing prohibits it. And the mongrel's difficulty isn't "None." It is just unknown until you draw another monster. See this comment from Mike.