Colette, you're the GM. Your monsters should act logically and attack weak enemies with tactics and stuff. Like, you're allowed to do that without giving all their info away to your players. Giving your players all that information removes a large variety of the mystique of fighting something new and powerful for the first time.
I'm sure, to give an example, you've never had an overconfident player roll a 19 against an opponent only for you to reveal its got a higher AC than that and shock the table about their chances.
This is not an indictment of your GMing style, I don't believe anyone is doing that, but I think most of us can agree that the system just wasn't intended to be played in the way you do. Mystique is an important tool in the GMs arsenal, after all.