[Aezeal]

Or make sure he's behind his charges so he doesn't get hit...
When I tried to use Hydras (this in Dom2 so it might not work as well) I'd put the Tamer all the way at the back line scripted Attack*5 (he'd get to the equivalent of the front of "your" formation box by this time), Retreat. The Hydras would be all the way forward on Attack Archers. Cavalry would run up, get poisoned, kill selves chasing down retreating Hydras. The Legion would then show up and find a nearly empty province.
After Construction 2 I'd stick a Longbow of Accuracy on him. Placed on the center of the formation box, scripted to fire closest.
Fun, but less cost-effective than simply getting the right gear on an Assassin and dropping all the commanders.
That, and the Hydras would get scattered around the surrounding provinces.

--> if you givethe hydra's guard commander that last problem wouldn't be there anymore.. and the hydra's (who can take a beating and deal damage) could then attack all those poisoned troops... seems good to me (nad just leave the commanders standing in the back, no need to move forward