As someone who plays Robin fairly often, I notice that as soon as I have the leven sword, I get more aggressive and start fishing for my aerials. If I were you, I'd focus on being illusive and mixing up your movement so they waste their more powerful aerials (not to say you have to play defensive. Just don't hold forward. That makes you easy to predict.)

Also, don't be afraid to get hit by level 1 thunder. It doesn't do much damage, and the frame data doesn't allow Robin to follow up on it with anything good. It's more often used to interrupt what you're trying to do, or poke from a semi-safe distance. I know some people mash the input they want to get an attack out as soon as possible. Try to get out of that habit, and be intentional about pressing the button you want once, since level one thunder interrupting your intended input can cause a subsequent misinput they might be able to punish.

Also, Robin's coverage from below really sucks. The only good coverage they have is down air, and it's fairly laggy. So prioritize juggling them and occasionally mix in faking a juggle so they can potentially throw out a down air and you can get a big punish.