[Saxon]

Hydra’s (at least in Dom I) are wonderful creatures. That poison cloud will really hurt an enemy army. I used to have two hydra’s with some cheap, low morale units, all in the same group. I put them as far forward as I could. They would advance and attack. The cheap units would die quickly (often due to poison), causing a rout. The hydras would flee, leaving a long cloud of poison. The enemy army would chase them, through the poison, taking lots of damage.

Recover hydra’s from neighboring province, mix with a few new cheap troops and repeat. Sure, they are exposed to arrow fire and you rarely win a real battle, but for wearing out big enemy concentrations or slowing invasions, they are nasty. However, DO NOT mix with your real armies, you will do at least as much damage to yourself. Consider them your special forces.