What happens when your opponent shuts down your arcfire? Previously said, Robin mostly relies on single hits or the occasional string to rack up damage. The fire jab is very slept on and should be used often for it is safe and has good knockback. Down throw to upair gets really predictable really fast, so I avoid using that too much. On the occasion where Arcfire is effective, go to town. Arcfire to fair or upair works great. You can also follow up fairs with more fairs! Down tilt to short hop fair also works at low percentages.
If Robin gets in a match where she can't Arcfire ever, then you are not going to do well at all. Robin NEEDS Arcfire in like every single of her match ups. Its her bread and butter, its like playing falcon without grabbing it just won't work. What I can say is you have to make Arcfire work.
fair into upair works at low percentages too. A good combo is Arcfire > fair > le-upair. After this you can fast fall and predict where they land into Arcfire U-air. This does a lot of damage really fast and your opponent might not see this coming at all.
Also didn't know about down tilt to fair works. You sure its a true combo tho, I mean have you tested it.
Also down throw is not a combo into anything ever.
Also fire jab is best jab, and can kill people early if they DI backwards trying to escape the jab which is nice, but don't sleep on wind jab either. It has its uses(just not a lot).