ROB vs. Marth is slightly in ROBs favor. It is incredibly in ROBs favor on stages like Delfino and Halberd where he can recover through the stage as well as on stages with multiple ledges like Japes or Norfair.
For Marth to win, he needs to over-b a lot, juggle a lot, and edgeguard more aggressively than he's used to.
ROB can't air dodge out of his up+b and has an incredibly slow fall speed, so double jumping off stage and fairing him then returning to the stage still gives you time to juggle him with a u-air upon his decent. If you time it properly, it becomes very difficult for ROB to land. The only way to gimp ROB is to dair him or hit him until his fuel runs out, so you probably won't get many of those.
If ROB ever hits your shield with a fair, nair, bair, dair, f-tilt, f-smash, or u-smash, you can punish him with over-b. At worst, the ROB will space correctly on some of the moves and you'll hit his shield (you guys know what to do in that situation, you shouldn't be punished harshly if at all). By simply running into ROB and shielding the moment ROB jumps, you eliminate his spacing options. ALL of ROBs aerials are punishable, and severely so.
ROB can't approach in this matchup at all. Your main approach should be walking slowly while powershielding projectiles or short hop air dodging through them if you have trouble PSing. Once you put ROB next to the edge of the stage, there isn't much he can do that is safe. You can now punish all his projectiles and can simply wait for him to move, then punish.
When ROB is falling, his falling speed is so slow that he can't do much at all. You can juggle him with u-airs easily, even if he's air dodging, and if you can get behind him you can do a rising nair through his air dodge and hit him with a fair. If you are in front of him, a spaced bair will force an air dodge then you get a free fair (or if ROB is in perfect position, aerial shield breaker).
ROB can really dish out the damage to Marth if he gets his combos going and Marth has to rely on chip damage via juggling and fairing off stage, so you can easily lose momentum in this match... especially since ROB can gimp you and you can't reliably gimp him (ROB has an auto-meteor cancel, so dair doesn't work unless they aren't expecting it anyways). As such, you have to be really careful about getting KOs when opportunity arises.
Whenever ROB throws out a nair when recovering, it's a free tippered nair for you if you're in the right position. At the very least you get a nair or fair on him. When he starts the nair, wait for the fire to start in front of him, then immediately jump and hit him with your nair at the peak of your jump. He shouldn't have time to air dodge if timed properly.
If ROB does a reverse nair, forget it. The hitbox ends behind him, he will air dodge and your only bet is to hit him out of that (keep in mind he can no longer fair if he's facing away from you).
ROB can also be easily KO'd and damaged when he's on the ledge. If he jumps and shoots a projectile, you can powershield and hit him back. If he jumps off stage and shoots a projectile, go back into edgeguarding mode. All of ROB's getup options are extremely slow, so if you space yourself to outrange his fair, you can be reactionary to every one of ROB's traditional ledge options. ROB can still clip a gyro onto the stage and then get up on top of it, booster through you with a fair, jump and air dodge onto the stage and d-smash, and other things like that but they are just harder to punish, not unpunishable.
ROB's projectile game is relentless, but if you keep your cool you shouldn't take much damage. Just focus on shielding.
If ROB is hitting your shield with a gyro, he's doing it on purpose. It chips away at your shield and he shield pokes with lasers. DI your shield town and towards you so this can't happen, or simply jump and air dodge to avoid projectiles. If you are patient, you can grab the opposing ledge and plank until he decides to get on the stage.
Im not fueling the fire tho
