“ Rather than making a baddy impossible to hit, consider giving it mega-hit points.
Super hard to hit leads to great player frustration. Allowing them to do damage leads to a sense of accomplishment. another variation of this would be to give the bad guy a forcefield that shows damage.You could also give them a mastermind ability that sluffs the damage of to minions who should be easy to kill.Once in a while a super high to hit encounter is ok ,but it should be few and far between.
Your mileage may vary”
“ The frozen wastes stretch for miles around. Something waddles through the snow. It's a penguin: An emperor penguin. It waddles slowly, meandering toward the sea. The ranger freezes. 'Stay very still,' he warns. 'Don't move at all.' 'What is it?' I ask, breathlessly. 'It's the most dangerous creature in the whole Yahoo Tundra, and that penguin's about to kick its butt...' (Sorry, Epi! I couldn't resist!)”
“ The PCs see a large oak tree, a curious species of yellow and black butterflies swarm around it. Upon further inspection they find that the tree is hollow and that there is an underground cave in it from which the butterflies appear. Will they inspect this strange cave?”