There are no difficulty levels. Also, what the bloody hell? The further you go, the stronger you need to be? What tragedy! Something completely unheard of in other RPG-like games [this is sarcasm, in case you don't get it]; things like "no level up/SL1" challenges could be possible on the likes of Final Fantasy entries and your average Soulsborne experience, not here. Flash news: your average RPG expects you to get stronger, or you get steamrolled even by mobs.
There are enemies that give you a decent amount of experience and that you can defeat in 3-4 hits, provided you take advantage of their weaknesses, this coming from someone who reached level 92 without farming stronger bosses, nor spending countless hours on this.