I put my strategy for dealing with the priestess and the rest in spoilers because taking her out sets the whole scheme up.
If you are a rogue or have one you can start with the Goblin camp on level 3(you will get to 4 fast). Hide the rest of the party inside the dark corner of the Priestess room then talk to her and get her to the room. Kill her with the whole party then stealthkill the NPC horde which will arrive later. They will not see you in the dark corner where you hide the casters and only dash out with your rogue to snipe the rest.
Once the priestess and her ensemble are dead you got a room full of barrels of exploding stuff. You need two for the drow priestess where you set them up near her so she looses half her health and the mob beside her is killed (necessary to avoid him running to the drum) which leaves the priestess and one extra mob. Manageable.
If you got the Blade of the Frontiers in your party as I did you can kill three Goblins on the far side of the captured Goblin priest in the cage adjectant to the Goblin priestess for extra exp by throwing them into the pit next to them. Now clear the warg cages by putting your melee near the two big goblins on the left(from entrance), your casters on the opposite side up the steps from them and your rogue starts the encounter with the bear putting the gate upon that bigger mob in front. Then eliminate the two runners with your casters and rogue while the "bosses" are tied down with your melee fighter (you could also use some barrels on them but it is not worth it as they have too much hp for my liking and you would need too many barrels). Just remember to kill the beastmaster with concentrated fire next to avoid mark on the bear and to despawn the spider. Then tell the druid to hide in the room.
Why this and not the Hobgoblin right after the Priestess? Because his death turns everyone against you and you want to pick the fights the way you want to. Now you are ready for the inferno. Get the Illithid corpse encounter cleared so he sits on his throne. Then everyone gets as many barrels as he or she may carry. Everyone and their dog needs one or two barrels alongside them. I gave the boss three alone and one near him so the chain reaction is guaranteed. There are more barrels above if you think you need more (I realized that after I killed the group so you usually should not). Split your party and move them up the ladder on the left side (from entrance) to the sparrows. Save, smile and throw a vial of fire down to the boss. Watch them all burn. In my case the boss had one hp left after running around like a burning chicken and I sniped him with my rogue.
After this everything is covered in ash and you can hide your party above while you snipe the rest of the goblins inside with your rogue and dash. You can even cheese the Hobgoblin body guard of the priestess in the temple by hiding your party at the enrance, stealthily climb the ladder once the oaf turns around with your rogue and snipe him to death from the shadows without any risk to yourself.
All was done with a less than ideal party of woodelf rogue main, shar cleric, wizard and warlock as I killed Laz in my second playthrough because I could not stand her in the first and I rather take the companions which don't cost me fun while playing.
