What does your OP have to do with challenge? Arbitrary rules that discourage play-styles you don't like is not 'challenge'.
e.g. Giving the boss DR 100/(insert anything here) is not challenge. It's a statistical obstacle that can be bypassed with more statistics. A five year old kid, or a fifty year old man, is as challenged by DR 100/(anything) as a Korean pro gamer would be. It requires a minimal amount of IQ, and zero reflexes, abilities, or game knowledge to make something like DR 100/(anything) completely irrelevant.
It's about as 'challenging' as the infinite secondary Con damage of Harry's poisonous attacks: it's made irrelevant via pressing a button. If that's your benchmark for challenge, there's something wrong here.