I have a Bristling Hydra on the battlefield, which states I can pay 3 energy and give him a +1/+1 counter and give him hexproof. My opponent has an Ahn-Crop Crasher, which states he may exert him and target creature can't block this turn.
My opponent exerts Ahn-crop crasher and targets my Bristling Hydra. In response, I pay 3 energy and give him hexproof. In response, my opponent played Lightning Strike, dealing 3 damage to my Hydra.
Wouldn't me activating the trigger ability of the Hydra on the Crasher still give him hexproof? I was told otherwise from the judge and my Hydra was destroyed. Was this a legal move?