Divinora
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If your heroes are well-leveled, there's no need to transfer units between them – in certain cases, this can lead to poor morale (heterogeneous troops, etc.). Besides, everyone will have to fight anyway. But if there are problems with leveling, then, of course, you can't do without transfers. For example, my Railag didn't turn out very well, so I had to transfer the Ents to Faidaen, and even then, he barely managed to win that battle. At the same time, Faidaen coped wonderfully without the Ents.
Here's another tip: first, you fight not with the Lord himself, but with the wall. The trick is that you need to hold out for 2 turns. It's better not to attack anyone during these 2 turns so as not to lose troops due to retaliation. And it's even better not to deploy all your units. Also, if you have a skill like "Catapult" (the catapult fires 2 projectiles at once), the battle with the wall will end before it even begins.
That's all I can remember off the top of my head. Yes, Railag has the most difficult battle – he fights last against the largest army. So be prepared.