From a low hillock in a vast plain, the entrance to the Noil Ruins is as unobtrusive as it is a gateway to wonder…
The Pelezians, or “clay people” were strongly bound to the earth. In fertile plains and valleys they lived their happy existence, peaceful, yet ever on their guard. Many nomadic attackers they defeated, until a different enemy came; their advanced technology and weapons (and magic) were beyond the skills of defenders. One-by-one their settlements fell, until the clay people were not more.
To the greedy students of the past, their heritage is but not forgotten.
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