It seems that the faster stacks attack the griffons, keeping them at the edges of the battlefield. The simple gnolls are easily defeated by the griffons, and they end up in the center.
There were only 4 flies available, not 6. I placed hydras in the first stack, gr.gnolls in the 3rd and 6th, and flies in the rest.
But the computer doesn't use them like that... It attacks, and they die on the counterattack. And the griffons just fly in faster. What's the effect of splitting the hydras?