In my limited experience, the more difficult battles generally have some sort of trick to them, but I'm sure you're smart enough to figure those out.
Other than that, if you are given a potion, odds are you're expected to use it during the game.
As far as resting all the time, that's one thing that sort of bothers me about NWN and other D&D CRPGs. In spite of the supposed urgency of your mission, it never seems to matter if you take a couple weeks clearing a single level.
Basically, your experience is a lot like mine. I often just lower the difficulty after a while, unless that makes the game too easy. There's no real difference to lowering difficulty and reloading/resting all the time. Just a matter of my own impatience.