The idea that a CR 2 dragon could wipe the party is potentially misguided, but it does bring up an interesting idea. Why not present the party with very, very under-CR'd opponents who nonetheless create a significant challenge due to the group's lack of tactics + the enemy's special abilities which happen to be shared by dragons? The worst case scenario is that they can brute force it (or run) anyway since the enemy's damage output likely isn't very big; the best case is that they learn how to counter some basic tactics and abilities in a safe setting. And if you really want to "punish" your players for playing a super-simple style, all you have to do is follow the XP and loot guidelines in the DMG for such a "weak" encounter.