Against which factions is it easier for demons to fight, and against which is it more difficult?

It's easiest to play against humans, and most difficult against dwarves and dark elves.

You're not entirely right. Playing against dwarves isn't that difficult, as dwarves have low damage, attack, and initiative. So, demons have time to summon reinforcements without significant losses.

Dark elves have little mana, and if imps move quickly and steal it, it might turn out that demons have a better chance.

It was correctly stated that light elves have the best chance of defeating demons.

Everything said is just my opinion.