Here ya go:
Hydras can attack with all their heads at no penalty, even if they move or charge during the round.
A hydra can be killed either by severing all its heads or by slaying its body. To sever a head, an opponent must make a successful sunder attempt with a slashing weapon. (The player should declare where the attack is aimed before making the attack roll.) Making a sunder attempt provokes an attack of opportunity unless the foe has the Improved Sunder feat. An opponent can strike at a hydra’s heads from any position in which he could strike at the hydra itself, because the hydra’s head writhe and whip about in combat. An opponent can ready an action to attempt to sunder a hydra’s head when the creature bites at him.
Each of a hydra’s heads has hit points equal to the creature's full normal hit point total, divided by its original number of heads. Losing a head deals damage to the body equal to half the head’s full normal hit points. A natural reflex seals the neck shut to prevent further blood loss. A hydra can no longer attack with a severed head but takes no other penalties.
Each time a head is severed, two new heads spring from the stump in 1d4 rounds. A hydra can never have more than twice its original number of heads at any one time, and any extra heads it gains beyond its original number wither and die within a day. To prevent a severed head from growing back into two heads, at least 5 points of fire or acid damage must be dealt to the stump (a touch attack to hit) before the new heads appear. A flaming weapon (or similar effect) deals its energy damage to the stump in the same blow in which a head is severed. Fire or acid damage from an area effect may burn multiple stumps in addition to dealing damage to the hydra’s body. A hydra does not die from losing its heads until all its heads have been cut off and the stumps seared by fire or acid.
A hydra’s body can be slain just like any other creature’s, but hydras possess fast healing (see below) and are difficult to defeat in this fashion. Any attack that is not (or cannot be) an attempt to sunder a head affects the body. For example, area effects deal damage to a hydra's body, not to its heads.
Targeted magical effects cannot sever a hydra’s heads (and thus must be directed at the body) unless they deal slashing damage and could be used to make sunder attempts.