[Yrkoon]

How about having a hydra tamer on holdx5 at the rear of the battlefield and hydras in front set to guard commander ? The hydra will retreat until they reach the tamer, leaving a trail of poison clouds in their wake. Enemy troops trying to catch them up will take damage without even reaching them for a few turns, them the hydras can shred whatever is left of the army.

Maybe not a great tactic against archers though. Unless they use up all their arrows, they won't follow the hydras.