Baldurs Gate 3 Baldur's Gate 3 - The Phase Spider Matriarch Guide (How to Beat on Low Level)

Guide to Kill Spider Boss Mother and Others

Use invisibility potions or sneak in any way to the eggs, destroy as many of them as you can, I believe there is 3 groups of eggs, if the big spider is near one of them try to pull it away, you could always take the spider to a ground fight or run far away so it leaves the egg pile and have other characters sneak and destroy them, anyways, deal with the eggs or you'll be dealing with Idk almost 20 small phasing spiders each with 6 health that will end up surrounding you and getting a flank bonus.
Then, try to snipe the phase spiders from far away while sneaking, from any place you can that doesn't aggro the boss spider.
Beat them, if you're having trouble beating just those, two tips, don't group your characters together, they use poison aoe missiles.
Second and most important tip, stand on the webs, despite probably getting enwebbed, having one or two characters standing on the webs is really useful, a familiar or anything like that would be even better, you can then destroy the spider web it is standing on, dropping it and forcing it to prone.
Use the same strategy with the spider boss mother, the fastest way of beating her is by going to this specific web bridge that has a hole below, obviously destroying that makes her drop into the hole, insta win for you.

Source: https://gameplay.tips/guides/8621-baldurs-gate-3.html

Baldur's Gate 3 - The Phase Spider Matriarch Guide (How to Beat on Low Level)