A Hydra is effectivly X monsters in one box... X representing the number of heads. Each head (or monster) has combat reflexes and 12 DEX.... that means that each head (monster) gets 2 AoOs per round.lukelightning said:"A hydra’s Combat Reflexes feat allows it to use all its heads for attacks of opportunity" is the disputed passage. This specifically is based on Combat Reflexes, which still limits you to one attack per instance (e.g. one attack on somone casting a spell, but if they move out of a threatened square and cast you get two attacks).
Others interpet it as "a hydra gets one attack of opportunity, with all its heads" which means all you need to do is distract it with an illusion and it'll use up its attacks on it, then you're safe for the rest of the round.
So, if the big dumb fighter moves to attack the Hydra, and moved out of it's 10ft. threat range, he provokes X AoOs... and then the dumb fighter realizes he's almost dead, and tries to drink a cure potion, he provokes another X AoOs... and hopefully (from the Hydra's perspective) dies.
Mike