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

I think the most challenging opponents for the Demons are Sylvan and Academy... This is because these factions have strong archers, and Demons themselves are not known for their initiative...

Elven forests are undoubtedly the most proactive in the game. But Demons are still in 2nd place. Archers can be countered with Chaos Cave Lords, new Cerberuses, Archdevils, and you might even get lucky and find the "Dispel" spell in the guild. Not to mention the "Berserk" spell, which will force the archers to attack their own defenders and receive a very strong counterattack.