[Quitti]

Shadow Vestals pretty much require some sort of bless to be useful against hydras - prot0 really hurts considering that hydras make plenty of attacks to get through your defence of 12 (+4 from shield). Being ethereal negates some of the hits - so they are probably your best bet, but if they get struck, they're probably done for.

For magic counters - perhaps mind burn or shadow bolts - both unfortunately restricted to your grand thaumaturgs unless you have astral hats or death staffs. Mind burn isn't very useful against the main head since it'll probably just regenerate through that, but it'll burn through the smaller heads and gives a better chance for your melee to chop up the rest. Providing that you can get through that MR14, which is kind of nasty.

Edit: Like Vedalken suggested - paralyze is good, but it's thau4 so you probably won't have access to it yet. If you have ench1 you should just use those thaumaturgs to go nuts on animate skeleton.