His texts is:

Response: After attached hero exhausts to attack Smaug, discard X cards from hand to reduce Smaug's (defense) by X.

The question is: Smaug's defense is reduced until the end of the phase or ever since response was activated?

Good question. I assume it is just for that attack, that's the way I play it, since it does not say anything about until the end of the phase, round, or game.

Yes since is responce is only work for current attack resolve

Oh yes, good clever