Something still feels off when I play Robin, like Levin sword ground smash attacks are too slow or get beat out by other people's attacks. Also I still a problem with people rolling too much, down smash only works sometimes, maybe I need to sidestep or roll cancel grab more. Speaking of grab I don't know how to combo off of down grab, people are usually thrown too far to be hit. I still have a problem with landing Robin's air attacks, I turned off up tap jump and I think that helps. I hope I get more advice and become better with Robin, thanks everyone!
Don't smash attack, to start off. Punishable smashes like Robin's are best used
only when a foe absolutely
cannot respond-- simplest example being a missed rest or ending up charging some laggy attack in the wrong direction. If you miss with a single smash attack, you're overusing them. Ftilt is an infinitely safer ground move, and you don't get screwed by a miss due to rolls or shields as easily.
Rollspammers are trickier to punish as Robin, but the key is that Dsmash is not some be-all-end-all answer as it might be on someone like Little Mac or Pikachu. If they roll towards you from a range, Dsmash can certainly punish that from time to time, but rolling
past you is best delt with via Ftilt, Arcfire, Dtilt, or a jab combo, depending on the location and character. Move as little as possible and their rolling will be far easier to punish, barring the 'roll away and projectile spam' sorts like 90% of Samuses or 50% of Lucarios.
Robin's grabs are, honestly, not that important to combo compared to most other characters-- Robin lacks for combos off throws other than the occasional Dthrow Usmash/Utilt/Uair. Uthrow them into an Uair at lower percents can do, while Fthrow and Bthrow are mostly for getting them offstage-- Fthrow might be used to loosely set a potential spike, while Bthrow's got nice KO power. That all said, Robin's grab is the game's worst, so it's not exactly the highest priority compared to say, Dedede, who has a game revolving around grabs.
As for aerials, the key is that you should expect it to fail unless a foe is in the air or in hitstun/helpless (typically the latter is due to an Arcfire/Arcthunder trap). Robin rather lacks for 'safe on shield' aerials, so focus on using them to juggle and to combo from Arcfire and such rather than trying to use them as an approach or the like-- that's not really Robin's strongsuit. Nair is your 'safest' aerial, while Ftilt is one of your safest ground attacks (only really beaten in safety by Dtilt, which has less range), so keeping those as a focus will help quite a bit. While Robin's Fair is fantastic for damage and all, keep in mind that it's considered her second-worse aerial simply because of how outstanding Uair, Bair, and Nair are.
Hope that helps a bit-- though I admit to lacking any solid advice for the projectile-heavy form of rollspammers beyond 'power shield and Thoron', which is certainly a case of 'easier said than done'.