on a thematic note. why is it immaterial and why does it die? and how does that relate to hydras?
This. I mean, REALLY.
"damn hydra which is pretty invincible" Hydras regrow their heads when they are cut off, they are very easily targetted and in fact become stronger the more they are attacked because 2 heads replace every one cut off. Why is it losing a head every turn anyways, it isnt being attacked. And while fire and cold or destroying a head may succeed, neither are being used. ANd hydras in general are water element
I suggest if you want a dark creature that is strong and untargetable but leaves after awhile, make it something like vengeful spirit who enacts his revenge then leaves the field.
EDIT small side note that bothers me, each hydra has an excessive point of health. they can only be damaged by fire buckler which would do 1 damage per turn until they die on their own. Therefore each hydra would have 2 HP left no matter what when they die. My brain just doesnt like it.