[thejeff]

Or if you don't want to spend gems on a bow, a few indy crossbowmen.
Again, place them part way back opposite the hydras. When he goes to stay behind troops, he'll be behind them.
If it's a really long fight, even the x-bows will run out of bolts and close, but there's no avoiding that. If you're going to use hydras in large scale battles, you need poison resistance or to just accept poison casualties.