See that's it, I don't know what is punishable. Do you have stuff that is directly counterable by your moves? For example, first page of this topic says you can Dair most of Sonic's Spin Dashing nonsense. ROB on the other hand can fast fall Fair you and it seems like the Robot has no landing lag. Plus ROB can throw out lasers. So that leaves coming in on the ground where Peach doesn't have range. ROB's Fair flame out ranges your Fair, so that doesn't work. If it outranges your Fair, that means Bair and Nair are outranged as well. ROB's Bair has landing lag, but the ROB I was facing rarely uses that. So are you telling me to air dodge into his aerials and use a Dair when coming out or what? What are some approaching options coming in, floating Uair?
Pulling out turnips Do Not work because ROB has his laser plus top and we no longer have lagless turnips pulls. You'll have to take a hit.
Fast fall fair you? That's silly. I think you're getting his nair and fair confused-- his nair is the spinning flame with little lag, his fair is the basic arm slash.
You're thinking in terms of range, but range doesn't matter if you have speed. ROB can only fast-fall nair you and hit if you're under him, or relatively close. ROB has to jump up in the air to do this. You must be falling for some sort of bait that he's conditioned you to do, because ROB's nair is very easy to predict from the set-up to the over-reliance when opponents get close-- that's when you quickly nair him before he starts the move.
And, if you can't get the nair out quick enough or you're not confident enough that it'll hit, just shield it and accommodate to what he does afterwards. If you have a turnip, you can throw OoS once he's done with his nair nonsense. If you don't, you have to sit tight and see what he does. He shouldn't be close enough to get a grab right away, but he can probably hit with an ftilt if you go OoS right after the hit. I'd recommend spot-dodging if he likes to grab, and just shielding if he tries to aggressively follow-up (which he probably is).
Fair is not a good idea for his nair, it's too slow and as such your hurtbox is extended while his hitbox is at it's apex in range. Uair might come out fast enough, but I'm not too sure, you'll have to hold tight and try that one out.
I don't know how you got airdodging into his aerials from my explanation! That's outright stupid. He won't throw any aerial out too far from the ground except uair (which you'll know when it's coming), so you'll be punished by any good ROB for doing that. It sounds like what you're having trouble with is getting in patiently. There's no use in using a move while going in-- simply shield, turnip, roll predictable zoning attempts, and once you're in, make him whiff a move on shield and you're golden!
If you don't get why you're golden, it's because there's only so much room on-stage to work with. As you come closer without getting hit, ROB is going to naturally move back towards the ledge. Eventually, he'll have too little room to be comfortable with, as laser runs out and gyro does too, and he can't roll back to buy himself some time when he's right at the ledge. That's when he'll start to fast fall nair you, sometimes do the spinny thing, ftilt, grab, yadayada.
Everything except for the grab is punishable with turnip OoS, and most of his options are punishable with some good timing depending on the move. And you should be dealing BIG damage to ROB once you get him, because he's so fat. His up-B has great distance, but it's hiiiiighly punishable because he has to stop his vertical momentum in order to airdodge, and that makes for great frametraps for Peach, which means a lot of damage.
Patience and turnips will help you in this match-up. Yes, turnips are punishable by laser, but only if you pluck carelessly-- when he uses his laser it takes time to recharge, so if you aren't confident enough to bait a laser and then pluck when neutral is far enough away, you can pluck:
- when ROB is offstage
- when ROB is respawning
- when the match has just begun
You only need turnips for getting in and edgeguarding, so you would only need to pluck them in situations where the neutral allows some leeway anyway.
If you keep plugging away at it, I'm sure you'll find that ROB is a gimmicky powerhouse and when you remove his over-reliance on his projectiles, it's a very balanced match-up, if not in our favour.
Or just go Captain and uair him a million times. Persevere and you'll do fine.