Fighters do fine against Hydra's, they have to suck up the pain, but they can be effective. Most will not have Improved Sunder, but you can still sunder heads, w/o it, just have to suck up the pain.
Charge, Spring Attack, Power Attack, Cleave, rinse repeat is an effective strategy, of course you cant be wearing heavy armor.
Charge, Mobility, Combat Expertise, Whirlwind can also be an effective combo.
Ranged Sunder especially with Order of the Bow, can be a Hydra's worse nightmare.
I Imagine a Whirling Frenzy Barbarian with Power Attack, Cleave, Combat Reflexes and Karmic Strike and a flamming weapon, buffed up with AC and Con Boosters, and the all important Shield Other could deal quite a bit of damage by himself.
Barb Charges, takes AOO, Barb Power Attack/Sunders Head, Barb takes AOO, Barb gets his own AOO from Karmic Strike Sunders another Head. Barb player argues he can get a Cleave Attack from sundering head, (which I imagine most DMs would allow)...the Barb wont last long, but he will Hurt the beast nastily.