[Taqwus]

And different orders as well -- the hydras should be good to go immediately and attack rear or archers, while the non-chaff units are further back with hold/attack. Even then there will be friendly-fire casualties without a full poison-immune force.

I used perhaps an even dozen in my first game as Sauromatia. Kept them on a flank, relied heavily on archers (both national and banefire) and somewhat less on normal melee troops.