Well, since this is wifi, it's pretty difficult to gauge what actually needs attending to, but i'll do my best.
I would suggest conserving smashes for opportunities to kill or when you're absolutely certain that they will hit, minimizing your lag and decay on your kill moves.
Try to keep anyone(especially ROB) from being below you at any time. Marth is fairly vulnerable from below, and someone could just fake an attack to get you to counter.
Since you're fighting ROB, who has a good spacing game as well, this forces you to change up your typical strategy to one that baits his attacks and/or allows you to attack first to get the hit.
Running into shield is fairly safe here vs ROB, since he didn't grab too much(I think) and spaced decently against you. Since Marth has so many options out of shield(DB, Fair, tilts, Dolphin Slash) this is a fairly versatile option, not to mention a few things that Marth an do with grabs.
If lag isn't excessive, you can wait for a counter when you see a startup on a move such as a Nair of Bair from ROB. Otherwise, it's not very likely that you can successfully challenge ROBs aerials.
I'm thinking that that's all right. Hope it helps!