So I reached a cave where there's a big spider and a "spider swarm"... the spider lasted for a couple of seconds, but I've been trying to kill the spider swarm for like 5 minutes. All my characters either miss the hits, or damage the swarm by 0 hit points. Also all spells are useless since the swarm seems to have a pretty high saving throws...

Anyone on that cave with the same issue?

Yup same problem, not even alchemists fire does anything. Official forum says aoe spells or flasks like fire or acid kills them. Not in my game. Nothing works.

Just went to steam and read about that... even if that's the case (and have to try it first), would be nice to have some ingame information about that. It seems as if devs are expecting people to know all the rules of Pathfinder before playing the game.

It's a swarm of small spiders, swords and bows and arrows aren't going to do anything, lots of fire is needed : )

Agreed, they said that they will fix that. The questgiver what was his name Barokko i believe will give warnings and what will help to fight the spiders. Not sure it will implemented via hotfix or a real patch. They look also into difficulty spikes and will adress them. Yes, and i tried a lot of fire, and surprise... it didn't work.
I mean common, small spiders. A big boot should also suffice. :p

Equip your torch and attack with that

In Pathfinder, swarms are completely invulnerable to normal single strike weapons. You have to hit them with AOE attacks, like good old Fireball or Acid Flask/Alchemist's Fire. If AOE doesn't work then it's a bug.

I had a pretty rough time with that fight, but i got it in the end, what i recomend doing is seperating out who the swarms are attacking and having your mages hang back out of range using the Acid splash cantrip.

That's if you have no other options and AOE isn't working.
Acid splash was the only way i could find to actually harm them.
It takes a while, but it worked for me, so it might work for you.

That might be part of the problem, most people here (me included) have never played Pathfinder before, so we might be a bit confused with that seemed to be a totally invulnerable creature. At least I read the devs saying they'll have the NPC alert that the spiders might be tricky to beat. Guess we'll have to get used to this, but then it might be difficult to see this isn't a bug without knowing that.

Don't have fireball yet, but tried acid splash with 3 of my characters and didn't even hit the swarm. Next time I'll try to have yet burning hands to use there, or then the alchemist's fire...

Most people here have probably never tried to disperse a swarm of bees with a machete. :)

I managed to hit them with 3 acid flasks and they did 1 point of damage. Color spray does 0 damage. That 1st level sonic spell did nothing (forget what it's called... something about a scream maybe). acid splash cantrip did nothing.

But a swarm of spiders? Sure, I have several resident spiders at home, and an old boot is usually the best way to deal with them. If I use fire or any AoE real life spell alike, I'll have to explain a lot to the firemen and the insurance xD

Pathfinder swarm is thousands of creatures in a 10'x10' square.

Burning Hands works.

I needed like 15 "fire flaskes" for it. Later on i learned, that you can battle them with torches, too....