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