This matchup in my opinion is awful. To beat fox in general you have to punish him out of shield and be able to anti air his approaches. Greninja is arguably the worst character in the game out of shield and overall bad at anti airing. The combo game on Fox also seems to be a bit unreliable mainly because downthrow dash attack doesn't work and dash attack up smash seems to rarely work. Unfortunately though the way to play this matchup seems to be just camping in shield and rolling or jumping away when an out of shield punish is impossible. If the fox is playing mostly on the ground going for grabs, dash attacks, and down tilts can initiate your combos and down tilts can confirm into up smash killing at 80-100%. If the fox is jumping all over the place you have to shield and deal with the tomahawk grabs because fox has no grab confirms. It is better than getter nair up smashed or raw up smashed. In addition to shielding, offensive anti airing should only be used when you know fox is gonna jump. Rar bairs in neutral are really good at stuffing out his Nair and other options but that's about it. Spaced fair is also pretty good but should be mixed in along your bairs. Other risky anti air options include raw up smash, up tilt (pivot cancel as well), uair (falling drag down as well), and nair. Maybe upwards forward tilt. Pivot grabs are good in this matchup and your bread and butter is down throw attack cancel bair. Up throw uair is only good at early early percents. Hope this helped, comment if you have questions.