For the fight in Grymforge you're talking about, there's also an opportunity to get some of the duergar on your side, which makes the fight a lot more manageable.

Also, the game never explains this, but you know the buttons in the bottom left of the screen during cutscenes? You can use these to trade with characters that don't normally have an option for it, to switch party members to sneak around while the one in conversation distracts an NPC, or to attack before a cutscene finishes playing out. Doing these things can affect the outcome of game events!