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I liked the Necromancer's Ascent and Phantom of the Throne campaigns the most, although the fact that Necromancer's Ascent is a rather easy campaign...
Yeah, if you survive the first 2 weeks of the first mission (when Jeddite attacks you), the rest of the campaign is a piece of cake.
Jelu's campaign is easy.
Gem's campaign – the first 2 missions are a bit tough, but then it's easy.
Kreg Hex – mission 2 (one Citadel castle against 3 "Castle" castles) is difficult, the rest are easy.
Yog's campaign – only the last mission is difficult, and if your hero isn't well-leveled (must-have: diplomacy + resistance).
I already said about Sandro.
Unclean Alliance – only the first mission of Yog is difficult.
All the campaigns are interesting in principle, but out of all the missions, there are no more than 5 that are truly difficult. It would be great if there were more challenging ones! :-)