Whenever Marth faces a projectile user in general, you have to do more work to get that first stock. My ROB experience is minimal, but I've played as him a number of times, so I'll just give what I know on the matter.
I'm sure you know that the walking/running shield is one of the best ground methods of dealing with projectiles (by holding down the direction you're moving in, then press and hold the shield button to slide into the shield), but in ROB's case, he may want you to block them so he can attempt to rush you from behind. That said, both his projectiles only hits once, and usually he'll followup with a running attack or grab after doing either one. Once you block the projectile, you have the option to drop and keep running forward, or cancel the blockstun by jumping and attacking back.
ROB's range and speed is about average unless he goes for his bigger commitment moves, and it's those ones you'll be focused on punishing the most. For the faster moves, aim for a grab or Dtilt depending on the distance between ROB and you, and whether you blocked the attack and he stops infront of you. If you know he's going to jump at you, try using Ftilt or Utilt to make him have second thoughts. The slower stuff is your opportunity to go on the offensive, SH/FH FAirs and SH NAirs and keep racking up the damage. ROB won't be the easiest target to gimp, simply do to his recovery. You're more likely to ko him on stage than you are off, so your FSmash or SB will be your go to for finishing the job (or Tipper FAir offstage).
Hopefully this helps
