I'm a veteran of these sort of games, and that should give me an edge, right? Wrong. The first time around, I too had difficulty; I will freely admit that. There were fights I reloaded multiple times, that I now breeze through effortlessly.
There is one mistake I made that, once I understood it, increased my successes, and joy, a lot.
In most of these types of games, what you bring to fight is the deciding factor; ie. Level, build, gear. This was most certainly true of the previous installments of Baldur's Gate. That still matters, but....
In this game, how you approach the fight matters *far* more. - Personally, I love this, it's less the computer-game approach and more the tabletop approach and I enjoy that.
First off: Scout the terrain. Keep an eye on the terrain, can you break things? Set traps with exploding barrels? Keep out of sight and position; can you get an edge? High ground works really well, provided you don't get knocked off. Don't just walk in like the mighty Dwarf that you think that you are, be smart about it. A typical Navy Seal doesn't ring the doorbell on an enemy base to kill everyone, either. Neither does the animal kind, but I digress.
Analyze your enemies.
Oh, Harpies sing? - Silence! [I kill you] They fly around? Then let's have archers on top of the hills instead of trying to run after them.
Goblins come by the dozens? Let's cover their base in grease, and set the place on fire. Or, take a defensive position and let them come to you; picking them off one by one.
Spiders hop around their webs to try and catch you? Set them on fire and watch them fall.
Archers hold the wall? Let's find a way to climb that wall unseen, and push them off.
