“ 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”
“ Dwarven Mountain Pony - a small, and hardy animal well suited to living in the mountains and highlands. Able to subsist on grass and brush, with hard hooves, and nimble feet. The dwarves use them as pack animals, and to pull ore carts, and to drive bellows for their furnaces. Never ridden as dwarves like to keep both feet firmly on the ground. As stubborn and tough as the dwarves themselves. Also only about 12 hands tall (1 hand=4 inches)”