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.
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.