Hammering away at Corbyn is stupid, imo. The normal response of people to attacks is to become defensive. Smith needs to present himself as having policies and beliefs close to Corbyn but with the support of the Labour party and a better chance of winning an election. Going hard on the attacking Corbyn line won't sway the voters he needs to sway.
This is, incidentally, why I still hold that having only one challenger to Corbyn was a huge mistake. They should have had someone who could run hard on the anti-Corbyn stuff, acted as a lightning rod, and let Smith come through the middle.