I have to agree Cruz played this badly.
The response should be “look I am not going to play this game of yours. I am here to talk about my ideas and qualification, not create some fake drama for you to exploit by trash talking any of the other candidates.”
You need to run for office! That would be an excellent response. Santorum pretty much says that every time they want him to denigrate other repub candidates. Of course, he had his chance in 2012, he and Newt should have made a deal and one of them should have dropped out after Florida. Romney won by dividing the voters.