It looks like many headed attacks on every attack the hydra makes. That attack could be as a standard action or as an attack of opportunity, both use all heads.
This does make it dangerous in melee but given its slow speed and lack of ranged attacks it is not very impressive out of melee.
If you are going to take it on in melee I would suggest opening up with a Ray of Enfeeblement as that will really take a lot of bite out of the creature. If you also have a way to entangle it that would hurt it a great deal as well.