Ranger "Refugees" - [5th campaign, 1st mission]

In this mission, there's a witch's hut near the city. This means that right from the start, you can send a secondary hero to check it out. If the witch offers a skill that doesn't suit you, you can simply restart the mission.

For example, I eventually managed to get the War Machines skill, and considering it has a 2% chance of dropping, and it's an incredibly powerful skill for the High Elves (especially in the campaign against the AI), it's worth it.