Well, I cheesed it with Astarion stealthily taking down the eggs and the phase spiders from range.
Basically, I turned stealth on all the companions, moved them far away so the matriarch and the 2/3 patrolling phase spiders wouldn't aggro on them, took down the eggs real quick with melee attacks, started the fight, made sure Astarion was out of their vision range and it was constant: ranged sneak attack => stealth, up until her death.
At some point I was curious about dipping the bow in poison (since the big mama spider is resistant to poison) and used the wyvern poison on the bow out of curiosity. Big mama spider got 3 shot with the wyvern poison coated bow... is it a bug or a feature I wonder?
