Hydras will almost always cast Meteor at the beginning of a fight. Multiple hits of those from a small pack of Hydras can wipe out your whole party in an impressively short amount of time. Spread out if possible so that it is harder for them to hit the whole party, or even better, try to interrupt their casting.
When facing Hydras, it is more efficient to nullify their damage via Protection skills or Stances such as Dryder's Defenses than to simply try to heal over it. Unfortunately, these skills are rarely available in Ascalon when one is low enough level for these monsters to pose a threat. As such, the best tactic short of interruption is usually good positioning, so their spells can only hit one party member at a time.