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The fact that there is only one main hero is a given.
1. The higher the hero's level, the stronger their army – this is a fact, and the larger the army, the better.
2. If you level up many heroes, the fastest way to gain experience, through chests, is spread across many heroes, which doesn't contribute to developing their skills.
3. Yes, expansion is faster, but if you encounter a strong enemy army, you will lose your army.
etc.
In general, I believe that one hero is needed for any map... and regarding skills, in my opinion:
1. Attack
2. Defense
3. Logistics
4. Pathfinding
5. Logistics
6. Ballistics
7. Wisdom
8. Earth Magic
I might swap the last two for Diplomacy and Magic Resistance.
My tactic is simple – a mobile hero with a good army.
As for mages, I'll say this – a mage against a warrior is simply useless when an expert-level explosion (50 spell power) will destroy dozens of wolf riders (50 defense + skill). That says something...