Secondary skills in Heroes 3.

Honestly, if we're talking about the game, it's boring to only focus on the main hero's skills. Developing just one or two heroes for the decisive battle is a one-sided tactic.
I usually develop two heroes for clearing the map and then for looting various treasure vaults, and two main heroes. Plus, there are heroes for defense and heroes for gathering resources. In total, there are 8 heroes on the field, plus those in the castles.
Each of them has their own set of skills. So, for the main hero, for the decisive battles as you mentioned, I choose four schools of magic, wisdom, intelligence, luck, and leadership.
And for those who gather resources by breaking into treasure vaults, the hero's unique ability comes into play. For example, for Valeska from the castle, shooting, leadership, and attack would be useful. For Verdish from the swamp castle, you can make her an expert in healing and increase her defense, then give her hydras, and with 5 hydras, she can do a lot in the treasure vaults and also simply occupy mines and defeat the guards. Of course, many examples can be given. The main thing is that you develop each hero for their specific task. And it's better to level up two heroes to level 10 than one to level 15. This will save time. And in this game, time is everything.