The hydra can attack with all its heads as an attack action. I meant to say that for each AoO provoked this way by the fighter, the hydra can attack with all 12 heads using the same AoO. Given its 12 dex, it can make 2 AoOs, or 24 extra attacks effectively.
Then, for each of the 12 attacks coming his way, the fighter can make 1 attack courtesy of RG? Bloody messy hell!
I can hear the ice creaking under my feet here, but the argument I'd proffer in this case is that "taking an attack of opportunity" is not the same as "making an attack". The very simple reasoning behind this is that "the attack action" is a standard action, whereas an attack of opportunity (obviously) isn't. As such, the Hydra doesn't get to use all of its heads for a single opportunity. Yes, I know what the Hydra description says, but bear with me...
The Hydra description states: "A hydra's Combat Reflexes feat allows it to use all its heads for attacks of opportunity." This isn't what the Combat Reflexes feat usually does, it's an effect specific to the Hydra which I would argue replaces the feat's normal effect.
As such, I'd maintain - should the situation ever arise in a game I was running - that this does
not mean "The Hydra can attack with as many heads as it possesses every time it gets an attack of opportunity", but instead "it gets one attack of opportunity per head instead of a number equal to 1+Dex mod".
At worst (or at best if you're the Hydra in question), it would mean "The Hydra gets to attack with all of its heads when it gets an attack of opportunity, but only once"; the reason being that in a very real sense it actually doesn't
have the Combat Reflexes feat. It has a feat that happens to be
called "Combat Reflexes" that does something different to the normal Combat Reflexes feat. It gets 1 AoO per round, per head; how they can be applied is somewhat up for grabs...
I appreciate that RAW support for this view isn't as solid as it might be. Given that I'm about to start playing a character who intends to start metamorphosing into a Hydra at the earliest possible opportunity, I'm very interested to hear different views on this
