Fauchard1520
Adventurer
I wonder if telling them the CR of their foes would work.
It does. I know from experience. The downside is that you're breaking immersion when you do this. That's the tradeoff. If you've got a player that is so far on the "game" side of things that they're unable to hear "this is a really tough monster" when you tell them in plain language, the only recourse is to communicate in game terms.