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You can immediately tell it's a complete newbie in a random game :) This brings to mind the words of Nebuchadnezzar, aka Captain, who claims that he has already studied everything in the game and knows all the tricks and features of the heroes.
The mechanism is simple - there are many Griffons, so they believe that the battle is already won for them. Consequently, they attack those who are more dangerous (in their opinion), with an adjustment for surrender, of course. In this case, the Phoenixes are clearly more dangerous to them, as the blocked Marks hit very weakly.
Although there is an element of randomness here. In different tests, the Griffons attacked differently. Sometimes they attacked not the Phoenixes, but the single Griffon that is above the Marks. In the third game, they didn't attack the Marks at all. In the maximum difficulty, they attacked them in one of the attempts.
Added 4 minutes ago
We either read with difficulty or it takes time to understand (C) :D? I wrote that in a REAL game, finding a food source is not a problem, and a book is not rare at all, so it is quite possible to save both angels. But I wouldn't guarantee that 3 angels and 4 horses, even with fifty Marks, will take down the maximum difficulty enemy.
I said above that the maximum difficulty enemy fell even without angels, although it was difficult. And with 3 angels, it will definitely fall. And one or two horses will survive.
As for the REAL game, sometimes there is a food source, and sometimes there isn't, it varies. Plus, even if there is a food source, what's the point? The Griffons will simply peck at this food source instead of taking a break, but the result will be the same. For this reason, it is not necessary to take anyone inside except for 3 attack stacks and single units.