In a bar not too far off the road, a man walks in. his boots clattering on the wooden floor. The bar is noticeably empty with only a few guys still around either drinking or chatting. Not too surprising since it is already the evening and they probably went home. The smell of smoke and liquor still hangs in the air like a fly caught in a web. The man in the trench coat pulls himself on a stool near the counter. His face covered by a bandanna and the brim of his hat obscuring the view of his eyes. The bartender, who had been cleaning a beer glass, looks up.
"So what'll it be, stranger?"
The man responds in a gruff and tired voice. "Dark and Stormy."
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