In all the runs I've done, we've never used PD for anything other than Alicorn (which was 1 group only) and even that seemed unnecessary as we just steamrollered it when it popped in several times Wave 3 with no PD available.
What does the Hydra have that's dangerous btw? I don't think I've ever seen anyone die to it and this is with 18-25 kills per run of that hall.
EDIT: To clarify, I've always gone on runs where mobs are stunlocked between 2-3 SCH's so I may not be seeing danagerous moves because of that.
The Corse can be a dick if Charm gets off but it's generally stunlocked whenever I've done it.