To use Robin effectively, you HAVE to be patient, observant, and devious.
There are some easy match-ups for Robin, and not-so-easy match-ups. Robin's biggest weakness is his sluggishness and lack of horizontal mobility. He can't handle rush-down. That said, Robin's best defense is to not get hit. Also, getting into the habit of catching Robin's discarded weapons will help tremendously. I have seen some Robin players trap opponents in Arcthunder upon shielding, then throw their tome at them, breaking their shield.
Personally, I'm usually good at dealing with characters whom rely primarily on close range combat; I just keep them out of arm's reach and keep hitting them with projectiles. If the opponent has a reflector or any other anti-projectile move, that doesn't mean you shouldn't use your projectiles at all--you just don't want to be
predictable with your projectiles. If I'm dealing with a camper, I simply camp them back. Right now, I'm experimenting on what projectiles Robin's swords can stop. For instance, I have found that a well-timed jab, forward-tilt, or neutral air will stop Samus's missiles.
Speaking from personal experience, the characters I struggle the most against are Zero Suit, Sheik, Diddy Kong, Little Mac, and the other really fast characters. That said, however, it is not impossible to win Robin's most difficult match-ups, for I have before.
If you're interested, see mine below playlists. I'm not the best Robin player, but I'm certainly not the worse, for I have been using Robin since the game came out.
https://www.youtube.com/playlist?list=PL5X3JQ8ayLWkQksu77V8WYUpKu7ImSRy1
https://www.youtube.com/playlist?list=PL5X3JQ8ayLWk7GgbRU9-sK_67QSj9-ewZ