“ 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 wild species, vinus homophagus, more akin to sea-grape rather than the terrestrial variety, is not a monster despite its fanciful name. The grapes, a deep purple color when in bloom, and oozing dewdrops of perspiration, like the most prized and delectable of drinking wine grapes, do however deserve their moniker. Wine made from this fruit, is deadly to most humanoids, as is the raw berry, if plucked and eaten from the vine. It is the unnatural chemical concoction found within the fruit's tart skin, which gives the man-eating grape its name. The chemical stew found inside each berry, functions as a necrotic agent, the same as found in some species of venomous snakes. The grapes literally eat the victim from the inside out, via cell death, dissolving organs and flesh in quick succession.
The tribes of Pra-Oohk Crater, from the jungles of Ghlush are known to sell the fermented 'wine' of this grape to merchants of distant lands. Sadly, the taste of the concoction is divine when first quaffed, and even worse, the man-eating grape wine will never detect as poisonous via mundane means, its horrid natures somehow masking all attempts. Luckily the man-eating grapes are extremely rare, and endemic to humid jungles.”
“ You find a patch of edible funghi. They taste well and all, no ill effects. UNLESS you consume some beverage, even 3 (three) days after eating it. Then you become really sick with pains, vomiting, all the fancy stuff. As a gift or good meal, it could be a cruel joke or to make sure the heroes spend their time focused on the mission. And the fine soup you had in the inn yesterday could have some in.
(Inspired through a real-world fungus. Was really used to cure alcoholism.)”