Actually, it's a combination of your level, the area you're in, and a bit of randomness that could send some really nasty stuff your way sometimes.
Each area has its min-max monsters, and there's no telling what will turn up at any time. However, the higher in level your party, the better chance you have of meeting monsters that are more max than min
For the Arnika road, therefore, 5th level is what most of us stop at, and then build up once we're in town. I'm not saying things will be easier after that, but you'll be better equipped to handle the challenges.
Nightowl2