Lure them into a kill zone where movement is difficult for them. In the room with the wall you break the opposite side has a ledge you can jump onto that takes you right to the phase spiders. You can have a character dash and have your ranged hidden with your luring character drawing them into a kill zone. You can do this with the tunnel you walk into the spiders lair but I had glitches doing this.
In my case I had a rogue hiding hiding in the upper ledge are and they pushed the spider into the blacksmith area as my fighter jumped down. My warlock and mage summarily executed one by getting into combat with two spells chunk health off and the spider only got one attack off.
