Absolute skills

That's what I meant by "mentor." For example, you need to capture a garrison, but it's guarded by demons who drain some of your mana. Behind the garrison is an enemy hero – a demon – and then the castle itself. But you're already out of mana. This is possible.
Another example: on the way, you encounter neutral creatures in incredible numbers. I've experienced this.
First, you don't have to attack immediately; you can find a well or, at worst, jump into a castle. And if it's really desperate, you can destroy some minor artifact.