I've never encountered such overpowered spiders in a game before. Even after they are on their backs and I stab downwards at them they still get back up and keep coming. They're more powerful than every other enemy I've encountered so far. --- Drink your milk!
The other most powerful enemy is the slave creatures. I don't know what they are but they take a lot of damage and they deal a lot of damage. Oddly enough, I found the creatures more powerful than the Necromancer Knights.
The cyclops were also surprisingly easy if you can get on a structure that is at their head level. It only takes a couple attacks. --- Drink your milk!