“ 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 court wizards of Angolterre have been working for several generations to protect the dynasty. They see to it that the new monarch's names acronymically refer to a rune of protection. Once completed, the rune will make the dynasty almost invulnerable in battle, wise in counsel, etc. Unfortunately the next name, the only one that starts with the right letter to complete the rune-name, happens to have belonged to the worst monarch in living memory. Not much chance of getting the current ruler to name his child that, or for the people to put up with it if he did! What's a court wizard to do? The magic requires that the plot be kept secret of course...”