The most difficult one is the Triumvirate. Two very strong heroes will come at you, and there's no way to deal with them.

Hmm, it depends on how you look at it. I played on "Veteran" difficulty. There were "astral gates" (or whatever they're called). I ignored the fact that I was supposed to (sort of) go save Godric first (the second mission, which I had to replay completely) and teleported through an outpost. I captured a Castle. I got lucky, and Narhiz and the Knight-Engineer appeared (in 1.6, (I don't know about earlier versions), you can buy heroes one at a time, so I bought a mage first, then fired him, etc.). I teleported through the second outpost using Zehir, and one of the two strong heroes attacked me. I fought him off with losses. It was a shame that I lost my last Rakshasa, but oh well. I have an axiom in my head: "All cities are on the way." I ran down, visiting only the places where I could buy troops. I captured a Castle. I saw where the second hero was coming from. I went to attack him. Damn it, he bought up all the archers right in front of me. I somehow managed to defeat him. The remaining two human Castles were captured without any problems.

And yes: I didn't have "Berserk," "Blind," "Charm," or "Phoenix," and of the resurrection spells, I only had "Raise Dead" (or whatever it's called).

In short: "Rush is king."

The difficulty wasn't in the missions themselves (as far as I remember, since I played the game with long breaks), but in the battles with the opponents. I prolonged many missions, so strong enemies attacked. And, since I didn't know what and with whom I would have to fight with the main heroes, I tried to level them all up. Although, if I had known that Railag (he didn't have any good magic, since I made Shadiyah the main character) would be in the final mission of the "Mages" campaign, I would have leveled him up differently. Because of him, I had to replay the battle with the Overlord several times (by the way, you don't need to level up Godric's "Guardian Angel").