If they like to jump, you can try a ledge hop back-air, but I don't think that trick would work twice. It's just sooo satisfying to pull off, so Ill mention it.
However in all seriousness, ROB's aerials have a lot of startup lag, and his forward-air does not have much range. Probably fast-fall neutral-airs, if timed right they autocancel into the ground so that would be one of the better options. BUT, it is not ROB's only option.
When you say ledge hop, you mean when you press jump off a ledge? ROB has another option to this. If you tap down on you left-thumbstick, you simply drop from the edge, jump out of it, and you will have a chance to fire a projectile. So if you ledge-drop>jump>laser, you can expect even some of the more strong opponents to be surprised, and if you are quick enough it is even possible to ledge-drop>jump>gyro, which is a very powerful method to start taking back stage-control thanks to it's lingering hitbox.