In a battle with Long Runner. I thought there might be another bug, not related to phoenixes. I also heard about creatures getting stuck under the Berserk spell. How does that work? (I think this is the right place to ask, but I don't know how to phrase it).

Long Runner can be beaten without exploiting any bugs.

The Berserk bug is used against Blue and Yellow Guy. There's a separate section about the Berserk mechanic in the Fizmig. To beat these battles, you only need to know that dragons under Berserk can sometimes get stuck and fail to attack a two-hex unit if that unit is standing directly in front of them. There are certain nuances, but they are not relevant to these two battles. The rest is about choosing the right combination.

P.S. While I was writing this, Marmot already answered:)