The most difficult boss encounter in a CRPG.

[Darth Canoli]

Dark Sun's last fight is pretty hard. And the rest of the game is pretty easy. At the time I first played, I considered multiclass characters in D&D games to be cheap. My first party was all single classed characters. Only two spellcasters. By the later stages of the game, they were laughing through every encounter and seemed absurdly overpowered. Then I got to the last fight and lost a good twenty times in a row. Never really came close to winning. Not enough spells. Everyone was a dual classed spellcaster in my next party...

Dark Sun is a challenging game if you let him.
Don't reroll, don't max out attributes or HP and it is challenging.

"By mid game" Every other game is a cakewalk by mid-game, in Dark Sun, there is still some dangerous encounters like Wyverns which can OS kill anyone, also, I remember the oasis battle which can be challenging and the demons far east and the Vroks later on, what's easy, on the other hand is the random spawns and if you abuse and grind, it can make the rest of the game easier.

Also, you said you didn't but most players do double or triple class in Dark Sun, some dual-class is alright, everyone dual class isn't and i'm not even talking triple class, it's alright is you're not looking for a challenge but don't complain later or brag about how stronk you are for maxing out attributes and then steamroll the game.

The early Dagolar's Lair is one of the toughest monster's lair ever created for a cRPG but of course, if you come back later, not only it makes no sense (you fled, you're not supposed to come back without an army) but it's just using a god mode.