I know this may sound patronizing, so I really apologize if it comes out that way, but you really just need to bear down on R.O.B. Just anticipate the lasers and power shield, and Fair over the Gyro (or just shield it or Nair it and R.O.B.), and keep going in. Once you're near R.O.B., just jab, or if you can, grab and Uthrow. Once R.O.B. is in the air, he really has no easy or reliable way to land against Greninja. Uair, Fsmash, Usmash, Bair, grabs, and Fair all catch him. You can throw a Shuriken occasionally, but you need to always be on him. Just tack on percent like crazy and then get a read for the kill (or Uthrow if they're at high enough percent). R.O.B.'s pitiful airspeed and floatiness makes him really easy to chase down once he's in the air. Like you said, the biggest problem probably is getting in, but it just takes a couple of precise attacks to throw him off once you're in mid-range, and then you can pretty much just go full ham. Just be aware of his grab range, though. Uthrow can kill us at around 135%, but stuff like Nair to spotdodge/roll and a well spaced Fair can help avoid grabs. Overall, I feel it's actually one of our easier MUs to be completely honest (all this is coming from my experience vs Mister Eric. We will be playing online later this week, so I may be able to provide replays and stuff if needed later on).