First of all, this strategy will only work if you have really leveled up your hero, and haven't just rushed through the quests. It is necessary that Railag is 100% leveled up with objects such as the arena, tower, crystal, and others that provide a constant increase in characteristics. In addition, it is very important to level up the hero correctly. There are probably two options. They differ in what your fifth skill is, because the rest should be taken without hesitation. These are:
1) Attack (Frenzy, Marksmanship, Tactics)
2) Knowledge (Magic Resistance, Clairvoyance)
3) Sorcery (Wisdom, Arcane Knowledge)
Unfortunately, we don't get to choose the fourth skill - it's Defense. I would prefer Luck or Leadership.
You can choose Darkness or Chaos as the fifth skill.
I focused on Chaos magic, as I was initially lucky with an artifact in the first mission (ice pendant) and spells (ice bolt and ring of cold). Be sure to take Storm Lord - in the later missions, we will be able to survive battles with superior forces precisely due to timely lightning strikes. Do not take Mana Burst - it's a useless waste of one level. I took Chaos Secrets because I was short on mana at the beginning, and Frost Nova. If you manage to level up Dark Ritual early, you can take Flame Lord instead of Chaos Secrets - it will also be very helpful in serious battles.
Now, hoping that your hero is really leveled up intelligently, I will tell you the essence of the strategy!
The key point is our audacity. Why waste time in dungeons when we have such a good army at the beginning, and our hero is an army in himself? We will immediately go to the nearest enemy Castle and take it by storm, before they overwhelm us with their huge increase in creatures. This will be easy, I assure you. In the meantime, Shadia will collect everything that is not nailed down: chests (gold, of course, we don't need experience anymore - we'll get much more from battles), resources; she will also capture various mines. Don't be afraid to let her run around alone - there are no patrols in the first few days.
After the first Castle, we go through the upper garrison to the second, which is located below and to the left. Along the way, we may encounter an enemy who has gathered all his forces in a panic.
Should I even mention that this strategy is not for those who like to use auto-battle?
We eliminate the enemy, trying to save as many Hydras, Witches, and our wonderful Assassins as possible. They will all be very useful during sieges, especially Hydras. During the siege, you can save archers by not deploying them, and split the Hydras into two squads - just in case. By the way, after taking the first Castle, we will be given a random fifth-level spell, but this is more of a nice bonus for the end of the game than a real reward.
As a result, if you hurry, all three Castles will be destroyed by the end of the second week. This means that there will be no patrols running around the map, and all the resources will be ours. All that remains is to prepare for the final battle. This will already be an act of your creativity. I will only advise you not to rush and collect several artifacts from the Dragon set - you can never have too much initiative.
When I completed this mission myself, I spent quite a lot of time on it because I decided to equip the hero perfectly and collect at least 50 black dragons for him. I just didn't know that such a weak enemy was waiting for me at the very end. And perfectionism required that Railag reach level 30 and defeat all the garrisons that prevent moving along the shortest paths between artifact shops and objects that increase luck/spirit.
That's all. The strategy is simple and dynamic, its implementation will be interesting and much more exciting than just running around in a dungeon and then hiding)