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For Humans, the biggest problem is the lack of money.
Axel is the best choice. But he has a poor start. Nevertheless, he will pay off in the long run.
As soon as possible, upgrade archers to marksmen. But at the same time, be sure to build all the buildings for the capital.
Then, always check the tavern and look for Elaine there. Assign all peasants to her to generate income.
It is essential to level up the second hero to at least level 5. Give him the skills "Expert Counterattack" and "Experienced Tactician."
While Axel explores the map, captures mines, and levels up, the second hero stays in the castle and, as soon as resources become available, transforms swordsmen into cavalry.
At first, there won't be enough resources for many cavalry units, but once all the main buildings are built, you need to train at least 5 to strengthen Axel's army.
Upgrading knights won't be possible at first - it's too expensive. It's better to build angels, but not buy them. Let them accumulate.
Against mages with the Darkness faction, use paladins; against the rest, use Isabel's knights.
I like this build: Logistics, Attack (be sure to include Retribution and Tactics), Leadership (very desirable to have a speed aura), Light, and Defense.