Well usually what works best for me is if I can launch the opponent and then start a juggle on them. Once they're in the air they're usually at my mercy. I can start that with any smash, sh-dair, dash attack or a full jab combo. After that it's just a matter of jumping after them and using uair, fair or bair.
I used to think that dair would be a good meteor smash but... Rosalina's dair really isn't a meteor smash at all. It launches off to the side at a low ~30 degree angle and if I get it wrong I've just launched them back onto the stage. The times I thought it worked as a meteor smash turned out to just be footstool jumping. It'd probably be great for gimping a character with a bad recovery but it's not reliable for that.
I think what causes him to end up behind me isn't rolling, though. Sometimes he just dashes through while I'm shielding, or he uses Jolt Haymaker and I shield, and when the Haymaker ends he's behind me. Both of these could also theoretically be punished with D-smash, but even when I get him in the air Ducky isn't easy to juggle. He knows exactly when to counter in order to avoid my uair, which basically tells me that I either need to mixup my juggles or come up with another strategy. I probably also need to FF more.
Overall he's reading me pretty well, which tells me that I need more variety in my strategies. Uair is always tempting but Rosalina does have other options.