So I'm at Sarevok. And he's obliterating my ass. I have zero chance against this guy.

I've tried everything. Buffed Minsc to the max: he still dies in like 5 seconds and is the only one who can make actual damage, but since he dies almost immediately, it's no use.

Magic spells and wands do 1-2 damage to Sarevok. Arrows don't even hit.

I also tried Animal summoning (so that they would distract Sarevok long enough for Minsc to murder him), but no, Sarevok massacres the animals like they were puny flies, after which he slays Minsc anyway.

I'm frustrated. What is it with these D&D games and their overly difficult final battles? It's not funny when the difficulty level spikes through the roof near the end. Icewind Dale 2 wasn't too bad, and that was actually fun, but Icewind Dale 1 and now this have ridiculously hard final battles. IWD1 I only managed to beat because one of my dwarves carried a sword that had a 2% chance of instakilling any opponent by turning them into ice. I have no such sword here, so I guess I'm screwed. I don't think there's any way I can beat this Sarevok guy.

Lots of people have beat Sarevok, including me.
Unfortunately I don't remember the details, though, but I seem to recall a hasted Kivan with the arrrows that do extra physical damage made mince meat out of him.
Use summons to keep him busy while you pelt him with arrows.
A stealthed or invisible fighter/thief may also be able to backstab him for significant damage.
Arrows of dispelling may weaken him.

And of course your party is buffed with every buff available?

Yeah, I finally beat him after all, although it was because of pure luck and stupid AI.

Branwen cast Summon Animals, followed by Sanctuary. Then I made him talk to Sarevok. Sarevok didn't attack because of the Sanctuary, instead one of his mages teleported to the entrance where the rest of my party was waiting. He was quickly dispatched. Then I made the wolves aggro Sarevok, and while he was busy hacking the wolves, I blasted him with Magic Missiles (the only spell that seemed to do damage). This was made easier by the fact that one of Sarevok's other mages dispelled all his buffs while dispelling Sanctuary. Then I cast Sanctuary again. The wolves bit Sarevok, Minsc slashed him behind the wolves with a 2h sword, and the rest of the party fired arrows and Magic Missiles. Eventually Sarevok fell.

Sarevok is simple on his own, but his companions make the battle very difficult. The strategy you used (dispatching mage first) is the exact same technique I had to use when facing Sarevok for the first time.
I also summoned an entire skelleton army. I don't know if you use TuTu or BGT, but in the original BG game you could have more than 5 summons, so I had about 50! That kept sarevok a bit busy while I killed off his friends :P

Well, it's the way of Baldur's Gate and I'm sure there are many difficult battle ahead, in Baldur's Gate 2. And BTW did you beat Durlag's Tower? I think that's much more difficult than Sarevok.

I'd say that we just spam Sarevok with summons and focus on Angelo and Samej first. Angelo shoots arrows of exlplosion so it's a good idea to drink some potions before battle (Actually my party drank whatever they have in the backpack); Samej is the pain in the ass so I cast Dispel Magic on him before he saw us and spammed him with the best arrows. He and Angelo fell easily, and then the battle is much easier as Sarevok was distracted by monsters and Tazok was very vulnerable to arrows.

Now I looked back, there could be easier ways. For example you can cast Cloud Kill on his friends which would definitely hurt them a bit, especially Semaj and Tazok as Angelo always teleports as soon as the battle begins. And once I soloed as an Inquisitor (I used tutu), which made killing those mages extraordinarily easy.

But all in all, yes it's a difficult battle and it should be, as it's the boss fight. And I swear that you are going to see lots' of such battles in Baldur's Gate 2 and ToB (especially ToB !!) which require lots of preparation and some luck, probably some reloads too. So good luck, and have fun!