Demon castle - Defeat the Demon girl

[Gatts]

Fire spiders deal physical damage, but I am not 100% sure
Anyway, demons are not resistant to poison at all as long as I know...
(I will look into the RUS manual and try to decipher it )

Keep green dragons - (EMERALD GREEN DRAGONS) - they deal PHYSICAL DMG.
put only RED and BLACK to garrisons.

So I have checked the Rus Manual (and I even didn't need to decipher it the color shows the type of damage):
Demons have all 50% Fire RES, (Archdevils have 80% fire resistance), Demons and archdevils have 10% physical resistance.
Fire Spiders:
ATT: 12
DEF: 12
Leadership: 30
INIT / SPEED: 6/3
HP: 27
DMG: 4-5 PHYSICAL
There is something what looks like "Defence +D" that is 3... I don't know what it is... sorry
Resistance : 25% FIRE, 50% Poison (poison resistance is handy to use poison cloud, as this is triggered each time any unit moves, even waits - then it is triggered twice so many times, so your units with poison resistance can fight them [enemy that are under poison cloud] with much less damage taken... literally 50% less )
So let say that the poison cloud deals only 40~80 DMG (what looks weak) , but you have 5 units and can wait with them (so we have 400~800 now) I had in average 10 enemy stacks when sieging castles (now we have 800~1600) the poison cloud lasts for 5 or more rounds ( 4000~ 8000 DMG ) and the poison cloud basic damage can be 120 ~ 240... making the damage to more enemy stacs (and they mostly always are grouped with 4 or more) 12000~24000 DMG in five turns (and even enemy can wait, also there can be more enemy stacks as 10, also you can use clone, summon, enemy can use hell gate...etc... all is adding dmg to the poison cloud) - just a hint
Did I mention that demons have no poison resistance?

Also Devilfish has 20% fire resistance (as the girl as well the demon use armageddon a lot - this could be helpful to know)

If you would like to consider to change from elves to dwarves, here are dwarven bonuses that can help you in this specific fight:
Giants can use mass damage skill (earthquake, or so) and non of demons are flying or floating (so all are hit by this skill) - this skill can be boosted with spell GIFT
Canoneers deal huge damage + have 10% fire resitance
Dwarves have really lot of HP

[kennec]

i like giants against demons couse their earthquake ability. very few of demon units are flying and avoid the damage. also demon units like to teleport in and out so giants slow movement is less important

[Tuplis]

On my playthrough (Paladin, normal) I used your setup except griffins and inquisitors instead of reds and blacks. Griffins are excellent tanks (they have more hp than leadership which cannot be said for many creatures), have good speed and init (with tactics they can reach the enemy on turn 1), unlimited retaliations and damage of 5-10 which means bless will take the avg damage from 7,5 to 10. I just made them fly in there and let the demons pound away on it pumping resurrect and letting the elven archers do their thing.

[Writhan]

Okay, succes!

Was a close fight, that lasted well over 20 rounds.

Setup:
Green dragons
Elven hunters
Druids (The old guy with the staff)
Archdemons
Fire spiders

The spiders basicly had sheep on them the entire fight. Druids was being controlled by demonesses always, so those two units was disabled for a long time.

I spammed Icy snakes on the currently strongest unit, and used drain soul on the strongest also, whenever I could. Used some shoal now and then, whenever I had extra rage available and waited for reaper cooldown.

Was a close fight, and all I had left was 1 dragon, ~30 elves and some 30 demon hounds I summoned at the end of the fight, to help me tank the last demons.

Uber fight.

Afterwards, I took towards the dwarven gate, followed the path and engaged the troll-boss guy. I nearly defeated him, he was on his last unit and BANG. Computer crashes due to overheat (I think). So .. I'll have to start over at him when I get back. He seems easier though.

Good luck with this demon fight. It's nasty.

[Infiltrator]

She was really easy for me (23 mage) I just popped a rage potion before going in, used the first death rage ability (whatever it's called) on the hellhouds, hypnotized them afterwords (couldn't prior to that), whipped a succubuss, then hypnotized her, then with the hypnotized succubi I charmed the knigts.. black dragons and the hypnotized/charmed guys did the tanking while I DPSed with my own demonesses, imps, cerberi and shamans. Pain mirror took out her strongest stacks quickly, and the damage they did was inflicted on her own (hypnotized) units.

In the end I lost 1 black dragon (excessive tanking I guess), 10 imps, 5 succubi and 15 cerberi.