Okay shouldn't be that much of a problem because you shouldn't really be that close. You have to keep your grabs flowing at all times during this match - up. Mixup shield - grab, standing grab, and pivot grabs. This way they are more pressured to be doing defensive moves than trying to hit you with Dtilt or DSmash.
DSmash though, yeah uit's going to hit you and that's just the facts. because sooner or later they are going to intterupt something you do with DSmash. Just try to Di out and away because unlike APika's Dsmash it sucks you in making it harder to escape earlier.
umm more tips.
Kepp pikmin on him, ROB gets pressured by Pikmin almost as much as D3, so if you can land pikmin on different body parts there isn't one way to get all of the pikmin off. SO they take damage while you beat on them, and what not.
It really helps if you get ROB in the air because they have a REALLY soft underbelly, meaning anytime you can get under them, they are pretty much at your disposal, they have Nair which can be air/side doged(you have to learn the timing) or WAC'd and and then punished apprpriately.