This is one of many results from the fact that this is a much more old school RPG.
Everything has a set level, and the game doesn't try to go out of its way to force you in the right direction: You have to explore, read and talk to people.
For example, when you finally leave Cyseal after defeating the evil boss, your main mission is 'find the white witch'. So you explore to find her. You find a barrier, but no way to clear it...or is there? I think inside Silverglen you can find out someone has a spell to remove it (which I found 'by accident' as I was exploring the forest). But you can also talk to your Imp at End of Time: he can tell you how to remove the barrier without the spell.
I just went through the mines. The death knights are ALWAYS invulnerable when you go there on your first go. They are supposed to be invulnerable. I used invisibility skills to sneak one of my party through the whole thing, leaving the other two at the portal near the entrance. When I eventually found a second portal with my sneaking rogue, I used the portal to have my party reform. The bit with the death knights isn't overly long, and invisibility skills/spells/potions really make you understand how little use sneaking really is in the long run.
I share your idea about rangers. They have good skills, but their normal attacks seem to be very weak, and I don't know whether that's because I'm not sneaking or their auto attack damage is just terribly low.
All in all, this isn't so much about character choices as it is about analyzing the information you have and (if necessary) doing some exploration to find your next goal. The game has some bugs and whatnot, but it's perfectly doable. Answers are sometimes just hard to find, and that was intended.
