This is the fight where you learn this game employs tactics in a fight. Examine, prepare, divide et impera, exploit enemy weaknesses.
If you carelessly stumble upon her nest, you're just dead in 1-2 turns.
If you don't just run through the dungeon, you probably activate and aggro just one phase spider group. This one group is manageable, even you're unsuspecting. After that, you know there is something. So you approach carefully and see more. You can prepare for more fighting and activate/pull more phase spiders without having the matriarch entering the fight herself. You can rest in between. Finally, if you approach the matriarch's location properly, you see the whole location, the whole arena. Remaining eggs, Matriarch, chasms, bridges. Prepare, spread the group, then enter the fight. Observe what happens. You see the matriarch positions herself on a silk bridge. That's an invitation to burn it, and this is the first thing you do. The matriarch falls down and is gone from the fight. Kill the remaining phase spiders. While doing that, the matriarch returns and positions herself on the next bridge. Burn it immediately! Continue with the phase spiders. The next time the matriarch returns, she positions herself on the last bridge. Of course we burn it. When she finally returns, she is alone and we finish her, being weakened by the fall damage.
This is how it worked for me after being instantly killed once upon stumbling into her lair. Nice challenging and not straightforward fight, and 2 or 3 team members temporarily downed while doing it, but definitely manageable.
