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

Theoretically, in the late game, all factions are roughly equally strong, except for the Fortress and Necropolis. The Fortress, perhaps, is the strongest late-game faction; its runes are simply too overpowered in the late game (specifically, the resurrection rune with the "fight to the last" mechanic: you attack the Fortress units, attack, and only one nearly defeated unit with 1 HP remains, and then, bam, almost half of them are resurrected at once).