I can't think of a situation where "tactical voting" ever worked. People cast "tactical" ballots in primaries all the time. I voted for Santorum over Ron Paul in Florida in 2012 because he had a better chance of beating Mitt Romney there, or at least press him, and because I viewed him as the most genuine pro-lifer, but to get people to do the enemy of my enemy thing widespread is not likely.
The way these things used to work was by Favorite Son candidacies. Popular Governors and Senators used to run candidacies in their states and would win the primary and control their delegation, and given the way this year is shaping up, it's not too late for that in some places. A lot of folks don't like Trump, but they don't like the other candidates either, and they don't like Mitt Romney. If folks really want to stop Trump, that may be the way to do it.
Truthfully, the reason Trump is hard to stop is because the average Republican is far more in agreement with Trump than with what we have been told a "Republican" is supposed to be, both by the Establishment and by the Movement Conservatives. I don't know if either party has been so out of touch with its rank and file as today's GOP is now.
Wasn't Newt Romney's main opponent in Florida?