"The Legend". Problems with the walkthrough.

I've been spending money all the time :(. I'm playing on normal difficulty, so the gold income is limited.

There are a few simple principles that allow you to conserve your troops. They seem obvious to me, but just in case:
1. You should attack each stack of enemy units with all available forces, taking the retaliatory strike with the least valuable creature or tank. The ideal tank is a dragon, which can be quickly healed.
2. For the attack, you need to choose a stack that the other enemy stacks won't have time to reach. The rest need to be blocked, distracted, slowed down, blinded, frightened, or put to sleep. Dryads are very helpful in this regard. Shapeshifting elves or wolves are also useful.
3. Your arsenal must обязательно include: vulnerability, dispel, healing, haste, slow, magic source, stone skin, fear. Magic is also needed for the warrior, by the end of the game: chaos - at least level 1, distortion - level 3, order - level 3. Max out all of the above spells. Vulnerability level 3 is much more useful than +2 to the hero's stats.
4. There are unpleasant opponents who constantly retaliate or simply have very large stacks that are scary to approach. They can also be defeated without losses. Summon a stack of thorns, bring it closer to the problematic enemy, let the thorns be killed, and then apply slow or fear to that stack. Then, using the Mirror of Pain, eliminate the entire stack in several stages.
5. Undoubtedly, the most dangerous opponent is black dragons, which magic does not directly affect. No matter how you try, even with advanced tactics, someone will still be killed. It has been noticed that dragons like to attack two stacks at once. You need to give them this opportunity, preparing tasty "meat" for them in advance. The dragons will try to block the archers, and teleport is very useful here. Teleport the archers to a safe place and shoot the dragons from there with dragon arrows.