A single attack (not just a full attack action, but one single attack) by a hydra consists of one bite for every head it has, including attacks of opportunity.
If you were to polymorph a martial adept into a hydra, or a hydra took Martial Study, or you were a psychic warrior with metamorphosis and a few levels in martial adept, or an egoist with a psicrystal with Martial Study that you intended to transform, which would be the best strikes for this?
Crusader's Strike seems like a good one. Heal your whole party at the end of each battle.
Steel Wind and Flashing Sun, for even more attacks.
Foehammer, for ignoring DR and getting up to +24d6 extra damage.
Drain Vitality, to basically eviscerate a foe with Con damage.
Tactical Strike, to move your whole party up to 60 feet, and deal up to +24d6 extra damage.
And those are just the first level strikes. There are more.
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By the way, blame an amalgamation of myself and Snakeman830 for this one.