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

Regarding the Inferno versus Elves matchup, specifically Nimus versus Wing, the chances are slim, but they exist. For example, if you play on the Outlands map, the Demon will have two native castles, while the Elf will have only one. Also, the Elf won't be able to develop their Special Ability, and they won't have anti-magic to remove spells. In that case, the Demon will likely win. But the Demon needs Darkness, Subjugation, Dispel, and Vampirism; otherwise, there's no point in even trying against Wing.