The hydras are pretty vulnerable to archer/crossbow fire. I bumped into a few in a MP recently and just 30 crossbows took out all the heads of about half a dozen hydras, leaving them in there 40 hp final form.
A air bless might work well with hydras to protect them from this. Up close and personal the things are lethal. Flying may well work on them, they can then close in on the enemy straight away.
Mass flight on a bunch of hydras may be as fun as it is on a herd of elephants. The problem is Pythium as no air mages I believe...