They're not serving you well here, so stop it.
One of the things that NWN does to protect you (or maybe only NWN 2) is that it gives you max HP each level. BG 2 does not do that by default -- it's possible to have a 12th level fighter with anemic hit points, just by the (un)luck of the roll. However, the config has an option to give you max HP. You need to do this. You may have some sense that you "shouldn't" because you play hardcore, but you are unhappy with the game, and there is a "make player happy" button sitting right there for you.
Also, have you gone into the pause options and set it to at least pause on enemy sighted and on near-death? Because I've been sniped at attacked on MANY occasions by enemies that were NOT hiding! The game had simply placed them behind buildings or their outfits were the same color as the background or whatever. The characters could see the enemy just fine, but I couldn't. Having the game pause was very important so I could assess the threat.
Likewise, having the game pause when a character hits 30% of hit points was also very important because that's when you run. These battles are not guaranteed wins. You have to be cautious in Baldur's Gate. You cannot be the arena champion at level 1, you cannot defeat dragons as a matter of course, and you should be really scared of vampires.
For me, nowadays I have a very easy time of BG 1 & 2. But I never learned about the counterspells, rarely use contingency spells or spell sequencers, and never use area of effect spells that might hurt my team also (fireball, web, cloudkill). So how do I have an easy time of it? My party never goes anywhere as a group. I always have a thief or summoned monster scouting ahead. I've used wizard eyes too, like them less, though. Having one or two expendables scouting ahead is VERY useful, because not only do you see what's coming up with little risk, but you typically force the enemy spellcasters to blow off a big spell or two while the rest of the party is tucked away at the entrance, waiting for news. If the spellcaster seems scary enough, I will have the party NOT advance, and throw some more summons or even lay down some traps. While I do that, the spellcaster's protections & buffs are timing out. The group only moves in when we've tested the waters and believe that we can survive.
As a parting example, consider the umber hulks you mentioned. That's not the last umber hulk ambush you will experience, by the way, so you need a strategy for entering rooms and falling back. In my case, even before I knew the umber hulks were there (before I had solved the Nalia quest), I had a strategy of leaving most characters at the entrance to the level, while a fire elemental or a summoned orc would move forward with Minsc opening the doors for them. If they got into a fight, Minsc would stay back and see how fast they died. If they were stunned or frozen and dead in a couple of hits, Minsc closes the door and runs. If my summoned critters seem to be putting up a fight, Minsc would join while the archers and/or spellcasters stay waaaaay the hell back and lend support from a safe distance. If things turn ugly, Minsc pulls back and the archers snipe the bad guys while Minsc is running. Repeat the process at the next door. In this way, you can survive just about anything, and you don't have to re-load the game 100 times until you figure out the "right" way to win the battle.
Play like your characters value their lives. Or make the dog stew.
