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P.S. I investigated what happens in Quick Combat when fighting the first Hydra in the Hive against 5 x 12 Vipers. In the first round, all the Flies attack the Hydra, just like in a normal battle, but in all subsequent rounds, the Flies don't make a single attack on the Hydras (probably due to the Hydra's ability to attack everyone around it). The same thing happens when fighting 2 Hydras in one stack against a Hive of 5 x 18 Vipers. In the first round, the Flies kill 1 Hydra, and then they "hesitate" to attack it again and don't attack the Hydras at all.
I think vbn is right. The Flies surround the Hydra from all sides and are afraid to attack because of the retaliatory attack on the entire stack. Perhaps this is something like a trick: 1 Hydra against 250 Mighty Gorgons, only with Blinding added.