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I just finished the campaign playing as the Wizard (I actually downloaded the first Heroes game recently and immediately started playing the campaign :D).
I used the following tactic:
1. Build a Dragon Tower as quickly as possible.
2. Start accumulating a small army of dragons (forget about everything else).
3. Acquire powerful magic like Armageddon or Storm, eliminate enemy heroes, and level up my main hero.
Then, using Armageddon, I defeated armies that were significantly stronger than my 5-15 dragons. In particular, I did this against Ironfist, Sleer, and the female hero (I forgot her name :D). It's really fun – I stand behind the walls and cast spells until almost the entire enemy army is destroyed. And if archers start taking down my dragons, I bribe them and run away. After a few such raids, the accumulated enemy army dwindles :D
Overall, it's a fun game, especially the music – in my opinion, it's better than in the other parts)))