I've been playing pretty much nothing but Robin since getting the game. Hopefully I can help a little.
Nosferatu is fantastic against people who have a habit of rolling behind you to go for a grab or attack. Just turn around and smack them in the face with your dark magicks. Outside of that, you can also use it to punish airdodge landings effectively. It's not especially worth using unless you're at high enough percent for it to scale up and your opponent is high enough for it to not be easy to mash out of, though.
Charge up your Thunder tome at any time where it won't put you into a disadvantaged position. Having a fully charged Thoron to punish airdodge landing is really powerful. Remember to hold down B for the extra kill power.
I find that Arcfire is really effective as a retreating aerial move if you can time it just right for it to still hit the ground. You need to use it just before landing back down for that to work. It's definitely possible to combo into FSair, but it also requires the move to connect properly and for you to follow up quickly. In many cases it's worth using FSair on the space just above where Arcfire has them trapped - if you aren't quick enough to FSair them while they're trapped, by the time you get there, that's probably where they'll be.
As has been said, using Arcfire to prevent advances also gives you a lot of time to charge up your Thunder tome. This can totally screw over characters Little Mac if you use Arcfire's limited uses wisely.
The spike on Elwind is risky, but also
really good if you learn to position it properly. Use it on people who like to recover low and right next to the stage for quick KOs.
The spike on DSair is better on opponents that recover higher, though.
Comboing out of Arcthunder is difficult as the move has rather long recovery, but it's definitely possible if the opponent is up close. Personally, I've been using Elthunder and Thoron the most - the former for edgeguarding and the latter for both edgeguarding and landing lag punishes - but Arcthunder is definitely strong. Others probably know its use cases better than I do.
Popping people off of the ledge quickly enough will put them in a good position for you to follow up with a BSair. Definitely a viable way to kill if they're at high percent. It's also rather safe for Robin to chase people off the stage with FSair; it's got a really nice hitbox and plenty of power and Robin can easily make it back to the stage with Elwind.
I prefer Robin's fire jab in pretty much every situation - the first jab comes out quickly and it's a great way to get people off of you - but it also means that Arcfire uses will be even more limited. It's probably true that the wind jab is better at low percentages, but I'm not too sure here as I don't use it much.
Robin's down-tilt comes out and recovers really quickly. Because of this, it's rather safe on shield and good at pushing enemies into Arcfire territory.
I could probably go on, but for now that's probably the bulk of what I've picked up so far. All the best.
