How to kill Black Dragons?

That's right, if I'm not mistaken, hiding units is prohibited in the official game.

You are not mistaken. Zveroboy made a mistake when he introduced this rule in the experimental tournament. He already regrets it, but it's too late. The good things from that experiment were forgotten, but the ban on hiding units remains and is still in effect. Even despite the lack of a proper formulation.
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The difference between online and offline play is perhaps more significant than the difference between Heroes of Might and Magic 3 and Heroes of Might and Magic 4.
I sometimes don't even keep up with the terminology.

Triple dragons are harder to kill than quadruple dragons; there's nothing to discuss... I haven't launched Whirlwinds in a long time; if they split into two or even one stack there, then it's useless with a Lifemancer...

Stranger, "Don't confuse me!" (C)
The saves you posted in post 92 can be opened with Whirlwinds, but are not visible for Equi (3.51)
The hero attacked alone, and the quadruple dragons were also in one slot. When attacking, the quadruple dragons split into three slots, which is typical for Equi, but not for Whirlwinds.
That is, there is a contradiction - the saves are for Whirlwinds, but the army composition is for Equilibrists.
Question: what's the catch?
It is clear that you can attack, or even play the entire battle with Equi, and then convert the saves to the Whirlwinds format. But why? - the answer is probably not in this. Somewhere I'm missing something... Where?