[Taqwus]

Put the hydras on a flank and forward, keep the non-immune troops back or on the other flank with hold/attack, fire or guard. The battlefield's fairly large, so don't mix your hydras with everybody else as you don't have to.
Get a nature mage, use the early Summon Horned Serpents ritual to get the expensive, Pythium-only hydras some cheaper poison-immune backup that you can quickly replenish, too.