Plan B for stopping Trump is shaping up: deny him a majority of pledged delegates, and broker a victory for another candidate at the convention.
But there's a fundamental problem with this strategy: it can't work for the candidate in second place.
Ted Cruz is not going to be able to win enough people over to his side at the convention, because he's, well, Ted Cruz. But he's the candidate in second place, so he definitely isn't going to drop out any time soon - especially if Rubio loses Florida.
And that means Rubio's delegate count is going to stay embarrassingly low. Even if Rubio starts picking up a higher vote share in April and May, Cruz's continued presence is going to mean Trump victories in plenty of winner-take-all states (and things get even bleaker if Kasich doesn't drop out). An embarrassingly low delegate count robs him of a lot of the credibility he needs to broker a convention victory. A loss in Florida would wipe out the rest.
Essentially, I see no plan to stop Trump that can overcome this.
Not to mention that, if this plan were to work, it could very well be that aegrescit medendo.