Nifft
Penguin Herder
Hydrae don't wear heavy armor. They can run 80 ft. as a full-round action. If it ends its movement adjacent to the Fighter, he's screwed.A hydra is just going to be target practice. As the fighter can out-move the hydra, a group of fighters could take it down in, as I see the numbers, about six rounds, with almost zero risk. A solo fighter could take down the same hydra in some hideous number of rounds by kiting... I'm going to say about 24-30 rounds, because of the very high fast healing.
I don't see how the Fighter can run away fast enough to allow him more than one attack every 2-3 rounds, which means his single attack must deal more than 40-60 damage every time.