Pest Control

To get this achievement you will have to kill the Phase Spider Matriarch before she can hatch any of the 3 clutches of eggs located atop three stone platforms around her nesting spot. Be aware though that her first action will be to teleport to one of the three platforms and hatch the eggs when entering combat.

The easiest approach is to have a party member with good stealth capabilities sneak onto the two platforms that are furthest from the matriarch and destroy those clutches first. The spiders do not seem to look around much so this will be easy. However, attacking the eggs on the platform closest to the matriarch will sometimes automatically initiate combat even if the spiders are not looking in that direction, so it is best to sneak party members onto that platform and destroy that clutch last.