yes, not many monsters deal slashing, and fire or acid damage. that could present another problem... ugh. why didn't I run through all the possibilities...? oh, well. I was hasty, and now I'm paying.
The Hydra, of course, was designed for PCs to tackle. not for them to play.

Maybe:
"a transformation takes 2d6 rounds, due to the way the werehydra's heads multiply from eachother."
Or:
"A transformation takes a standard/full action, but only one head is present after transforming from humanoid form. it takes 3d10 rounds for the original number of heads for it's form to grow after transforming."

and maybe I should replace "original" number of heads with "standard" number of heads.

Also, yes, a very drunk and stoned wizard. But weren't all lycanthropes made ORIGINALLY by wizards? Aren't they out-of-hand experiments?

But, yes. very drunk, and very stoned.
As I felt from fatigue when I drew a picture of a werehydra mage a few months ago (which was the inspiration for this). I'd post it, but my scanner's conked out. Grr...