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I didn't have a single dragon throughout the entire mission. And I don't think they're needed! I was taking out powerful armies with Rainlag, using Phoenixes and Rain. The armies consisted of 4 Assassins and 3 Minotaurs per slot.
And units of levels 5-7 are just cannon fodder in this mission. It's better to leave them to guard the cities – anyway, in battle, Warlocks will immediately bombard them with ice shards, and they're useless.
In this Walkthrough, my Rainlag also has a Phoenix. I'm not sure if I'll finish it. At home, I'm already playing the 2nd mission. There, I only need to level up Shadia, visit all the upgrade locations, and find the Tear of Ashi. I decided that there's no point in leveling up Rainlag's summoning skill. I'm not using the Phoenix much now, because Meteor Shower and Earth Shock are enough. And there's also the general Stone Skin skill. As soon as I start playing this mission at home, I'll drop this Walkthrough. There, I gave Rainlag Chaos, Attack, Education, and Magic skills. He was born with Defense. Considering that the main enemy in the last Campaign is a demon, I'll try to teach him Exorcism.