“ 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”
“ A city is being plagued by a swarm of winged mice. The vermin can get anywhere, and they can easily avoid their normal predators by simply flying away from them.”
“ Urbants-Creatures that move quickly around on their massive arms, and their 'legs' dangle about from their torso, at about elbow length. They can move fairly quickly, but not as fast as an obese human, and can 'jump' fairly high. Their 'legs' are very powerful, and can kill with one blow. The bad thing is, Urbants cannot block attacks at their heads, which have relatively no neck, from the length of their legs being so short.”