I'm not sure wiping the floor with them will work out very well,
It's always worked for me. The only player I ever lost that way, I was glad to. Usually, it's fun, a little bloody, even exciting. One or two surviving PCs stands over the flaming ruins of the battle and everyone else rolls up new characters. Usually, no second lesson is ever needed.
With my most recent group, there got to be a point where combats were just dragging on and not everyone could agree on what to do. Well, they came upon a group of ogres and decided to ambush them. "They're just ogres," became famous last words. Two ogres, two ogre barbarians from the MM, and an ogre adept killed one of the PCs and everyone took heavy damage, including the wizard. Afterwards, they worked out that they could easily have taken out the ogres if they had bothered to detect magic first, if they had even bothered to look at the ogre's equipment and guess if they were "just ogres," to have a backup plan in case the wizard got charged, and to ponder whether going to toe to toe with a fully healed, 9' tall giant was really a good idea even for a barbarian. By the numbers, it should have been a cake walk.
Not long after that, they started a long spree of attacking things out-leveled for them and winning. Most recently, they picked a fight with an ancient wyrm and managed to survive two rounds of combat and escape with no fatalities other than their henchman.