Greninja is a TOUGH MU and it's one of my least favorite. I didn't even like it in smash4 and he wasn't nearly as good. As far as shadow sneak offstage goes you can try either parrying it, countering it at high percents or challenging it with Fsmash. Parrying it is pretty difficult though. I've tried it a few times but for some reason, I couldn't get one even when I knew the shadow sneak was coming. For countering, you should wait until Greninja is at high percent to guarantee a kill. Your opponent will definitely feel pretty dumb. Fsmash works because many times when they use shadow sneak offstage they pop up beside the ledge so you can space out a good Fsmash.
In neutral I don't have many solid tips. What worked for me is just getting used to the MU and polishing the way I approach. Marcina mains, myself included, tend to throw out moves to wall out opponents but against characters with high mobility this can be risky. This is especially true with Greninja mains since their main gameplan is to dash dance around, bait out moves, and weave in with his quick combo starters (dash attack, grab, Dtilt) for a good punish. Try not to throw out moves carelessly and don't use dancing blade too much since it's pretty laggy. Mostly try to get a sense for how your opponent dash dances and when he likes to approach. What I've noticed is opponents that rely on bait and punish don't always get right in your face since it's difficult to react if you dash at them. So if you are, say, under the right battlefield platform, they would probably dash around the left one. If you get closer to the middle of the stage it will force them to react and they will have to either decide to attack you or back off and wait for you to make a commitment. This will give you a sense of how they make decisions and it will give you a better idea of how to read them. Mostly, stay patient and play the long game. Don't try to force approaches or you will just get blown up.
The other two ways Greninja mains play are overly aggressive or campy with Nspecial. The overly aggressive variant I find easier than the bait and punish type. Marcina's kit is very good at poking people who try to attack all the time. Try to keep center stage, and use quick moves to box him out. The campy ones are annoying as well. Most of the time their decision making is pretty flow-charty. They'll throw shuriken, wait until their opponent jumps at them, and either intercept in the air or dash back and whiff punish. Try to avoid playing into their gameplan and don't panic and rush at them when you get hit by a couple of shurikens.