I summoned the Bristling Hydra and it was killed instantly before I declared I was making it Hexproof. Could I have prevented it's death if I had said it was Hexproof before the other player cast his spell?
The additional reason for asking this is due to the energy production of the card and the energy requirement for playing the ability.
When bristling hydra resolves it provides 3 energy and the ability requires 3 energy therefore this question has some additional implications about how the spell could potentially resolve i.e. does the energy resolve in time for the player to use the ability or do you need an additional source of energy