Having a natural affinity for the underdog, I'd like to think about what Romney should do to win. However, the problem seems to be that the main obstacle preventing Romney from doing that is Romney himself. Even without all of his gaffes, he is simply a terrible candidate for virtually every swing state.
The only idea I can think of that would give him a Hail Mary would be to repudiate the Ryan budget proposal and instead call for dramatic cuts in military spending to make incremental cuts to social spending more palatable. Obama would be unable to successfully attack him on this for much the same reason Romney has been unable to attack Obama on foreign policy. Obama could attack Romney for flip-flopping, but given Romney's reputation I hardly think this would do any marginal damage. Of course, Romney's a hyper-militarist so this would never happen.
The Ryan plan isn't what is going to kill him. What is killing him is the second tax plan he released before the MI primary. He should have stuck to the broad outline from the 59 point plan that provided answers for more pressing tax decisions, and ended with a call for a long term reform plan lowering rates and removing deductions. But in MI he released specifics on the lowering the rates, and left the removing deductions blank. No the numbers don't add up and when you add them up for him the situation gest worse (see Clinton speech, he can either... or he can... The one part I saw of it, ironically).
He needs to show some flexibility/willingess to compromise on the specifics of the tax plan, provided that it lowers rates and the tax burden remains relatively the same. He just has to be careful to provide some room for revenue increases once in office, though. That will free him from his pre-MI plan without having to repudiate it directly and at the same time show that a willingess to compromise and look at alternatives that may be a better idea.
The then needs to hammer his five point plan, hard in the debates and advertising.
Finally, he needs to articulate a "sensible" regulation package to replace Dodd-Frank with, otherwise Obama will just keep hamering him about returning to the Bush years. Romney needs to be seen as offering an alternative path forward, not back to the future.