The main tactics so far have been either (1) a summons-heavy full assault, or (2) a summons-heavy pincer movement followed by retreat, trying to separate Drow from spiders, or (3) a fire-based tactic (let's not talk about that one...). In every approach, my priority has been to take out the two Ghost Spiders ASAP, then mop up the other spiders, finally the Drow. Also in every approach, Imoen has been blasting away with Horrid Wilting and Emotion from the rear while a buffed Nalia tried to divert some attention from the frontliners. Basically, I can never take out the Ghost Spiders before being crippled by the Drow to such an extent that the Fallen Devas, Flamestrikes, and remaining Spiders then slaughter my party. I'd split my warriors into a first-phase and second-phase group - one to kill the Ghost Spiders and probably sacrifice itself, the other to hang back in order to later face the spellcasters with their crucial buffs intact - but even with two Swanmays, there's not enough melee power for that to be effective.

Since the enemies in this battle have high magic resistance, the only fire-based tactics which may work against them is a tactics including Sunfire and Dragon's Breath spells (i.e. spells which bypass MR). Your levels re too low for DB; if you don't have enough number of Sunfire spells either, you should forget a fire-based approach.

In general, and in every battle with spiders, you should give priority to Whisper Spiders, because they are almost as dangerous as Ghost Spiders but are much easier to kill.

ADHW doesn't bypass enemies' magic resistance. First, have Nalia fire one spell trigger consisiting three lower resistance spells on one of the two Ghost Spiders and then cast spells which can be blocked by magic resistance. It may help.

I doubt that splitting the warriors can be effective because you will lower your total amount of damage per round by keeping some warriors out of the battleground. Anomen is now probably high level enough to cast Free Action on your party members and hope that the enemies' remove magic spells won't have a big chance to dispel it. Imoen can also use the humble Larloch's Minor Drain to interrupt the enemy spellcasters' spells (remember that it bypasses MR).

It's certainly a difficult battle. The fact that you haven't forged any of IA's items makes things even harder for you. You could at least forge Phosphorus in chapter 3. It could make a meaningful difference in your offensive power.Try different tactics and tell us when you finally succeed to win so that we also celebrate with you.

Spiders and drow at the same time = game over, at least for me.

I send one strong fighter (barbarian, good saves - the most important thing in this fight for sure) to trigger the spiders. The point is to make them waste one or two of their improved webs and lariats. Then I drag him back to my party (if he's webbed or mobbed - reload) when my mages casting fireballs on arriving spiders. Then all my fighters attacking one ghost spider, trying to succesfully save against those improved web (if your protagonist is webbed, don't panick! do your job, but if he's webbed and they start hitting him - reload). Mage cast greater malison and right after that R/C cast firestorm on everyone, spiders and party (127 prot. vs. fire - spells and postions casting and drinking last). If all works like planned, the weaker spiders should drop like...flies. The strongest should be seriously wounded. One ghost should be dead, finish whispers after him and then second ghost and you're set.

As Sikret mentioned above, Sunfire bypasses MR. You mentioned that you were overwhelmed by Fallen Devas, so what I would suggest is to memorise a few Sunfires (2-3 in total may be enough) and use them to disrupt the Priestesses' summon spells: they usually cast Summon Deva in the first round (the 2nd round if they cast True Sight in the 1st round), so your mage would have to be amongst the first sent in to cast the spell (should be Hasted).

Another good time to disrupt their spells via Sunfire is when they all begin to cast Summon Bears, unless you have a Death Spell handy (memorized, Vhailor's Helm or Skull of Death). In general, powerful Priest spells take a long time to cast making them easier to disrupt.

Most of the enemies have high MR (except for Killer Spiders which have none... not sure about Lesser Spirit Spiders), hence Lower MR should be used if you want to use offensive spells aside from Sunfire. Sikret has already mentioned the trick of 3 X Lower MR in a Spell Trigger, and you can also set a Spell Sequencer with 3 X Flame Arrow (3rd lvl spell) to be used once MR is lowered. Area effect offensive spells can probably only clear out the Killer and Lesser Spirit Spiders.

The Whisper and Ghost Spiders should be your top priority. Whispers first as Sikret mentioned, since they are easier to kill. Ghost Spiders MUST come next else they might summon a new wave of spiders. If fighters get confused by the Whisper Spider, get Anomen to cast Greater Restoration, which cures the Confusion... you cannot afford to wait it out. Also, if I remember correctly, Ghost Spiders need +3 weapons to hit (but I'm not sure of this). Use Enchant Weapon to help you out here, since they last 24 hours... and +3 weapons have better THAC0 bonuses than +2 weapons... just mentioned this as your Anomen's only wielding +2 weapons. Anomen can be invaluable in this fight with Righteous Magic + Draw Upon Holy Might.

My personal choice is to deal with the Spirit Spiders next. They have decent MR so damage spells may not be effective, and they have a chance of Holding/paralysing your characters. After that, try your best to kill the mages if possible, as after some time they will cast Horrid Wilting... by then your Abjuration buffs would have been removed by the Ghost and Whisper Spiders.

I have done that encounter a few times now but don't remember 2 ghost spiders , do you get an extra one if you are higher level?