Shrinkstools are a powerfully enchanted toadstool, typically found in enchanted forests or other secluded magical locations. Watch your step.
The Slow Death Cap looks, to those who don't know what they are looking at, like an edible mushroom, but eating it is one of the most unwise things anybody could possibly do.
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