Cruz and Trump are running for President not their ancestors and only one of them was born in the US.
If Trump wants to blow smoke over a non-issue, Cruz should find a way to blow a little smoke right back at him. And do it with a smile. Talk about lederhosen, haggis, and Trump as a relative newcomer, not that there's anything wrong with that.
As a tactical matter, Cruz should have anchored his story a bit more in his mother's family long ago. This is a battle for perceptions, and framing is important. The public thinks of Cruz as the son of a Cuban immigrant. Cruz would be better served if he were known as the great, great, great, whatever grandson of farmers and millworkers from Delaware and Pennsylvania. In a perfect world, Cruz would have an ancestor who froze with Washington at Valley Forge or charged with the Irish Brigade at Fredericksburg. Paint that picture, and I defy you to find any American who will question that the great, great, great, whatever descendant is not a natural born citizen.
Whether Cruz's family tree has any such dramatic vignettes, I don't know. They may have just been farmers, laborers, and factory hands. But you can sing a song about that as well. Paint the picture. Sing the song. That would settle it for most people.