Rubio wanted to wrestle the pig. Now he’s gonna find out the pig always wins...and he loves to wrestle (not calling Trump a pig, just saying that he fits that role in the old fable).
I see your point. However, I think what Trump is doing is exactly how you would want to respond to schoolyard bullies. Fight back with ridicule and at least bloody the bully's nose. Bullies hate being ridiculed and they don't like people who fight back. They will quickly move on to other targets.
The media and the establishment candidates have tried to do to Trump what always worked for them in the past. They would ridicule and marginalize anybody who dared upset the status quo.
They did this successfully with Sarah Palin, Ross Perot, Herman Cain and many others who dared to be a maverick and upset the apple cart. But Trump...Trump fights back. And the bullies really don't know what to do about it. They've never been up against this kind of opponent before.