Play as Axel.

Well, the main army is obviously Knights and Isabel. At the very beginning, I try to upgrade archers to the advanced level, along with the Knights (who have shields and protect the archers from ranged attacks). This is a pretty good combination for clearing out neutral creatures, but there's a big drawback – you'll be severely lacking in ore (20 units per upgrade for both buildings).

So, at the beginning, you go for advanced crossbowmen, and you should always have Knights with shields standing nearby. You assign one peasant each and clear out the neutral creatures.

Then you switch to Knights and train them in large numbers. Skills depend on your preferences; for me, it's attack, luck, light, logistics, and training.
P.S. I play as the Academy more often :)

Definitely! You need to level up your second hero to at least level 5. Give him Expert Counterattack and Experienced Tactician.
While Axel explores the map and captures mines, and also levels up, the second hero stays in the castle and, as soon as resources become available, converts swordsmen into cavalry.