usually for kills vs a rob, i just try to look at his damage, and send him as low and far as possible, so i can attack with a Down Air, followed by a UpB, its really awesome....
so when you have the right damage, look for an attack that can send him down in a diagonal way, because that may be the only way you can spike/meteor him for a kill....i find no other way...
as far as the on stage fight, i find diddy pretty much rules, if he knows how to perfectly shield all he throws, then he wont have a problem...when he gets close, rolling away works, rob cannot really follow diddy that fast to catch onto his roll... though sometimes i fall for the usual, Bair, and get hit by the Front Hit-box, forgetting that the attack covers almost all his body >_>...
and as for approaching, bananas are extremely good, you just have to be careful if you leave them around, i hate when they grab your bananas and throw em at you...