I dunno, it seem to me like Larian give us opourtunity to cheese the fight ...
Its up to us, if we right now want to feel strong and heroic ... or if we just want to run through enemies like nuclear warhead.

And i find it fine ... for people who do have some sort of self-moderation.

Because she's a phase spider with an intelligence of 6! Dumb as a post is working as intended.
"I sees food, I gets close to food, I bites food" probably gives her a headache, too much to ask that she understands why "The food in a skirt burns me! The pale food bites back!"
Is she tho?
I mean, if you search that cave, you find few diaries that was written by some spider-cultist (Llolth i believe?) that was dreaming about turning himself into some spider-like monstrocity.
There is even list with some ritual. O_o
Im not quite sure now just from the head if you find some corpse, that points out that s/he was not quite sucesfull ... or if s/he was and s/he is now spider Matriarch. O_o
But im not quite knowledgeable of DnD so im not quite sure how would such ritual work, and if caster's intelligence would really be reduced to only basic reflexes and insticts of regular spider. :-/
Nor if that is case for Phase Spider ... since as far as i know, they arent exactly "natural" in this world.