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The most challenging campaign for me turned out to be the fifth one, playing as Fyndan-Fedyu.
I completed the first four missions easily, without focusing on magic or spells.
In the end, I couldn't complete the fifth mission where I faced Kolya-V.Vampire. Fedyu didn't have any clones, resurrection spells, or elemental summons. I had to replay and acquire all of those. I replayed quickly and swiftly defeated Kolya.
In the sixth campaign, I properly developed my magic skills, and only the fourth mission was a bit challenging.