Ummm...none of those options? Like pretty much every DM, I often struggle to build challenging but fair encounters because my players are so damn unpredictable, bless! I don't think it's the CR's fault, though, I think it's just that the game is super complex and what may be a deadly encounter for one group due to composition, strategy, luck, etc., might be a breeze for another. Last week I ran an encounter that one beginner group walked through like it was nothing; I am running the same one with a different group today and we'll see how it goes.