“ 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 PCs have been traveling for a long time down the same stretch or round with no sign of anything suspicious. As they cross over to the next hill they see a gigantic splater of blood with a bleached dragons scull in the center of it. It the dragon skull's mouth is a tattered backpack with something wiggling inside of it.”
“ The PCs are making their way from village to village across a rural hinterland, when they spot a weird sign along a farmstead's fence, with an arrow pointing to the actual farmstead.
'CRAZY DANCING HOES!!'
If they investigate, a rather sedate local farmer, explains to them that for a mere two coppers, they can witness the show for themselves.
The show involves six ordinary, animated hoes 'dancing' on a makeshift stage, as the farmer accompanies them by slapping his thighs to keep the beat, and playing on his flute.
If asked how the trick is accomplished the farmer demurs, not wanting to share his 'secret'.
'If you can get Old Man Purkiss to tell you how he gets his cows to spout poetry, I'll tell you how I make my hoes dance.'
In reality, this minor encounter can lead to the PCs discovering that some localized, magical effect is active in the area. All kinds weird phenomena seem to occur in these parts.
Perhaps the PCs have finally found the ley-line of mana energy that they have been searching for! The party's wizard gets excited.”