One could give the tough enemies some dark aura or something similar to show it is different from normal enemies.
Maybe even have a cutscene/popup/skit about how dark aura'ed enemies are dangerous and should be avoided.
After a long break from rpg maker I suddenly got a craving for some mapping. Though, purely for fun as I'll never end up with a finished game. I'm still missing events, but the treasure delivery trolls are supposedly on their way.