Everything about heroes.

In this sense, I'm a black sheep. I don't use secondary heroes! They just get in the way. I do everything with the main hero. And if there are several main heroes, then I have to worry about the others so they don't get killed, so I hide them in shrines and finish the mission with the main hero.

This is a big mistake! Just try playing with multiple heroes. It might be difficult at first, but once you learn how to build chains and interact effectively between heroes, the results will surprise you! For example, why should the main hero go and capture a weakly guarded Castle when someone else can do it? Not to mention the quick transportation of troops from the Castle.

I always take experience, and gold only if I'm completely out of money or there's no more experience to be gained (see my new signature).

Mistake number 2! Experience should be gained in battle. Why does a level 15 hero need extra change (500-1000-1500 xp)?

I developed a super ability twice - the first time for Gotah (Pure Rage), and the second time for Zehir (Arcane Knowledge).

In most cases, chasing a super ability is a waste of time and effort. There are many more useful skills! For example, a barbarian doesn't need a catapult, but to get the super ability, he has to take it! This prevents him from getting a rapid-fire ballista.

Added after 11 minutes

and Logistics, because it's very useful to move quickly around the map and see the exact number of creatures in enemy armies. I advise you to do the same.

I agree 100% here! Take Logistics while you can! Logistics is not only about the speed of completing a mission. It's a real saving of your troops. What's better: to capture an empty Castle or a Castle with a bonus? Success means being in time!