Duck and cover was a hallmark of the Romney/Ryan campaign.
Meanwhile Clinton's "victim" isn't smelling so sweet...
You have a point about Trump firing back when attacked. Sometimes, it's necessary.
The problem is that it amounts to playing defense only -- Hillary is getting off scot-free, when she should be getting hammered for her unpopular positions on immigration, Wall Street, etc... Nobody is going to vote for Trump (or against Hillary) because of this story. It's essentially just damage control, which doesn't advance the ball.
The better way to have handled this is for him to have delegated the counter-attack to someone else, and spent his time focusing on Hillary. But she's had the best week she's had in awhile precisely because nobody is talking about her.