Lionryo's Heroes 3 MF Videos

Yes, and in the first round, the white units with a speed of 10 just stood there and did nothing, which was interesting.

That's the point: two-cell units don't fly into the nearest wall. But they do fly into two walls. And they also fly into one distant wall.
Although, of course, 3 walls is a lot for a Conservatory. Two walls are enough, but you need more Fairies.

Okay, what about the Medusa?

Everything is fine there too. Medusas will do better walking behind 2 Fairies through a wall than attacking with a single unit. That's the whole secret.
When playing as Moon, there are several standards that, once you know them, you can anticipate what to expect from the monsters. The rest is just a matter of technique.