Cruz has to find a way to get up to the #2 slot in the polls. Then he may have a real shot at putting a dent or possibly even overtaking Trump. He will have to do superbly in the debates along with a good effective ground game. Tall order from where he is now but it can be done.
You’re correct— what I find curious, is that everyone expects him to go after Trump for votes. Carson has 20% and is a much easier target. You get Carson’s votes, and then Trump is manageable.