Walkthrough tips, Discussion of Campaigns [Heroes 5]

But where does the magic come from? There aren't any particularly powerful spells there...
And his druids are weaker than any magic... plus, you have to get to his castle first. His monsters aren't particularly weak either.

No, you're wrong. I learned some great spells in the early missions, which helped me a lot in completing the final mission for the Haven faction. (Try conquering the Dungeon castle and going through the garrison to that castle; I did it!). So, you need to level up your hero well at the beginning!