Well the Hydra's aura is Poison.

So if you get anything that helps with Poison it will help.

Try forging "Snake Rings" and a spell called Poison Ward (Enchantment 4) or Serpent's Blessing (Enchantment 7) will help ward your entire army.

The best bet with Hydra's is to keep units that can stand up to a little poisoning in the same battle and try to keep your normal human units in a different army