XMorsX wrote:
 I cannot find the warden template in the pfsrd. That being said, 7 players will probably end a CR 9 encounter too soon to consider it an epic fight. A CR 10 will be more appropriate, maybe even more according to your players and the monsters being used. 
A more powerful creature may one-shot them though. Consider toning down the hydra and adding minions in order to make the fight more difficult.
EDIT: I found the template. 
Honestly you can take a normal hydra cr 4, and put it in a room with 1 entrance and 1 exit on opposite sides. This can be done via cave, maybe a thick swamp where trees block the way etc, or even a dungeon etc.
The room is 40 by 40 or something large like that. The entire room is flooded by dark water. The top of the water being floor level with the entrance doorway pathway that the PC's walked on, make the entrance so only 1 PC at a time (so 5 foot by 5 foot doorway) can even see in the room, you can make the path behind it curve. So no one other then 1 PC has LOS or LOE. Forcing the PC's into the room.
So the water is technically pit.  The PC's can only see 5 feet down into the water before total concealment.  Make it about 40 feet deep, double move for the hydra. In the middle of the room put a small island something like 10 by 10. The room has no walk ways so the PC's have to either "fly" or swim over the water. You can put a "key" or something in the Hydra that pops out as it explodes as your PC's kill it. ETC ETC I am sure you can come up with a ton of reasons why they HAVE to do this fight.
The Hydra uses the water to move and uses it 10 foot reach to stay in the water and to attack PC's using the pounce. Using this tactic over and over until it gets hurt enough it swims down and waits out its fast healing to regen it's life.
Your PC's will have to ready attacks. Either 4 PC's on the island and 1 PC in the entrance and the rest in the water.
No one wants to be in the water, make them all roll swim checks if they go in once per round, due to the deplorable condition and snags and gunk in the water it is considered rough water DC 15.
If they are foolish enough to all go in the water, it is feeding time. Try having the Hydra grapple and pull the weakest character down 40 feet into the water.
If you succeed your party will freak.
This is arguably a CR 5ish challenge due to terrain, but it is a very very deadly fight for the ill prepared or too eager of a party.
It's seems your party is a little higher but even then with these conditions it makes for a rough fight. You can either add a fitting template to the hydra or add a second hydra that is waiting for the first to distract the party striking when the time is right.
Either way this scenario will make them think of water and hydras never the same