He should ignore Huckabee and stay on his own message - tell people of his accomplishments.
The more Huckabee attacks Romney, the more he will look like a Chihuahua (sorry for a doggone reference) barking and nipping at the heels, and the sooner he becomes just a nuisance and an annoyance, not worthy of a vote or time to listen.
I think my point is that he has to knock out Huckabee, he cannot beat McCain unless he takes Huckabee down first. So he needs to focus his effort there first.