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For me, the most challenging ones were:
Battle Cry of Freedom - a terrible mission with a huge number of units; it was very difficult for me.
Dark Messiah - the beginning is very difficult, especially for the hero, and sometimes you can't even survive the first attack by the Necromancer; I spent at least two days on it!
Diplomat - I cursed this mission because of the annoying condition of not fighting with mages; there was even a time when they invaded my lands and almost attacked my city; fortunately, I managed to complete the task, but I was very stressed. The easiest ones were:
The Last Flight of the Falcon - the first scenario I completed and my first game as Demons =) (I even remember running around with Jezebel when she still had the skin of Biara).
Trial of Agrath - it wasn't exactly easy, but it was of medium difficulty; the main thing is not to make mistakes and clear the map.
Alliance - also a very simple scenario.
Maahir's Gambit - one of the easiest scenarios, in my opinion; the Necromancer has no chance, especially in Hammers of Fate. Zoltan with 2,000 skeletons simply fell apart. I can't even remember the others; I didn't have time to play Hammers of Fate properly.
Battle Cry of Freedom - a terrible mission with a huge number of units; it was very difficult for me.
Dark Messiah - the beginning is very difficult, especially for the hero, and sometimes you can't even survive the first attack by the Necromancer; I spent at least two days on it!
Diplomat - I cursed this mission because of the annoying condition of not fighting with mages; there was even a time when they invaded my lands and almost attacked my city; fortunately, I managed to complete the task, but I was very stressed. The easiest ones were:
The Last Flight of the Falcon - the first scenario I completed and my first game as Demons =) (I even remember running around with Jezebel when she still had the skin of Biara).
Trial of Agrath - it wasn't exactly easy, but it was of medium difficulty; the main thing is not to make mistakes and clear the map.
Alliance - also a very simple scenario.
Maahir's Gambit - one of the easiest scenarios, in my opinion; the Necromancer has no chance, especially in Hammers of Fate. Zoltan with 2,000 skeletons simply fell apart. I can't even remember the others; I didn't have time to play Hammers of Fate properly.