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

in terms of danger...
1. Sylvan
2. Academy and Dungeon
3. Fortress and Dungeon
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*Sylvan is probably the most "weak" faction... even Necropolis doesn't attack them (due to low initiative)... although they are protected from Dryads and Dire Elves...

Added after 1 minute
*but with proper development... for example, a Rusher... Inferno doesn't care who their opponent is...