“ 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”
“ Inside a mad scientists laboratory the PCs find a short creature with smooth, oily and whale like skin. The head is faceless and smooth also. But hundreds of white eyes on the PCs from the fat belly of this creature. The PCs are about to kill it as it speaks to them in a feeble but intelligent voice, asking them to pour the contents of a green vial on it. Will they do so? And if so, what will happen?”
“ As you emerge from the shop in the alleyway, not-too-distant clanging and stamping makes you wary. Further investigation reveals a massive Balgrian protest march flooding the main street, banners roaring about the inhuman conditions in which this ethnic group is forced to live. City guards stand helplessly by, beating up the odd protestor, but unable to hold back the flow. Onlookers throw vegetables at the Balgrians, and shout abuse.”