He spotdodges and then does his d-smash, so why are you constantly running into it? That's the oldest R.O.B. trick out there. If people still do that, that's funny. Make him think you're going to run directly towards him, short hop and then use a dair. Simple as that. Of course if he learns from his mistakes after that, then you're screwed. So mix it up. Don't always go and SH > dair. If he does decide to shield instead, be ready to grab him.
Also, if you're at a distance, R.O.B. mains like to fire their lasers or shoot their Gyro. So if you're at a distance, get ready for either of those attacks. They're pretty obvious. Keep your eyes on R.O.B. You'll notice him tilt his head if he's going to use his laser. But once you get so used to this, you can just have your PSI Magnet ready. One more thing. I heard you should keep him in the air.