Wait, are you assuming it can be done or have you actually done it? Because assuming doesn't help...
ROB is pretty even overall. He's pretty good at being annoying and pressure if you let him get set up for it. Lasers and gyro will be spammed, so be ready. His ftilt and fair have pretty good range..be especially careful offstage because ROB can chase you offstage with fair for a gimp kill. Keep in mind that his recovery is not infinite, so get him offstage and keep him there. He has a spike and reflector, so just keep those in mind. ROB has problems killing...expect nair and Fsmash as the main ones...usmash can kill, but it has horrible hitboxes...dsmash shouldn't kill, but know that it CAN.