I would prefer Cruz, but Trump, Trump/Cruz, or Cruz/Trump would be acceptable.
The problem right now is to sweep away the posers, the mediocre, and non-hackers like Carson, Christie, Rubio, Kasich, and particularly the moron, Bush the lesser, who are sure-fire losers.
I agree with that.....but to answer the headlines question, the only person who can topple Trump is Trump probably - and if he does topple for whatever reason, it will probably play right into Cruz' hands. There are some areas where Cruz is very much more reliably conservative....and I don't see Cruz bringing them up - but if they come up for some other reason, it will be very interesting to see how this plays out.
The other thing is....will Trump attack Cruz like he did Carson - when Carson surge in the polls? If he does, I think that will play into Cruz' hands. IF he does......