Level 40 cannot be reached at all atm. Guess the achievement is a relict from before they adjusted the XP system which was changed quite a lot since Early access.
Maybe it's possible after the next patch with the new dungeons. But anyways, all relevant skills are maxed out at the end of the game anyways and as you said - there isn't much of a challenge anymore.
Balancing is not a strong point of this game unfortunately.
I don't quite agree. I'm a Min-Max kind of a guy, so by the time I hit level 20 or so I started to Steam Roll. Had a pretty decent fight up until then though. I love it when a plan comes together, and suddenly you start having an easier time. The alternative would be things like level scaling, which I loathe, so no thanks. I would't have a problem with a higher difficuly setting for people like Kordanor though, so you can keep your challenge up. But as it is now, IF you've got a grasp of the systems you can build some really powerful character combos. If you HAVE to play like that, it would be less fun for many of us. I like to find every artifact, kill every monster etc. This will (hopefully) always make the end game a bit trivial, but that's part of the fun.