“ 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 local fungus-breed explodes if hit or fallen upon. If there is any conflict between inteligent enemies,both sides are likely to use it to their advantage.”
“ A civilization which constructs of irregular shapes constructed of a light metal, heaped together so that they stand on each other; these structures rattle and bend in the wind or at a push, but ultimately hang together except under heavy force (such as cannonballs, falling stones, floods)”