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

Well, it can vary... Imps are usually hidden, and they steal mana, taking four times less than their own amount. That is, only 100 imps will steal 25 mana, which is quite a lot for a Warlock – enough for one enhanced Shock of Earth (with a decently developed Sinister or with Secret Knowledge). These creatures have a large growth rate, and if they also have a dwelling, then it's even better... And the Town Portal has low defense... Although, of course, the Town Portal's demons won't defeat them easily; it will take some effort, but still...