Well, this seems to be a micro-guide for the final battle, as I understand it.

I just don't understand one thing - the Destroyer won't reach the enemy on the first turn.

And there's another point I don't understand. What prevents the Haven from using the same tactic if they have Sun Knights?

A thousand apologies, the Destroyer won't reach, but the Devastator with the 3rd tactic reaches perfectly. It's just that everything was written in a hurry and in 5 minutes from memory. And it's not Stone Skin, but naturally Petrification. In fact, the enemy (in this case, the Haven) loses 2 Shimmers and a Sun Knight; they can only attack the Devastator due to his "Terrifying Visage" ability, and he is protected by petrification and is invulnerable. And we won't get anything from a similar maneuver by the Sun Knight except for his quick death.