One response is Greninja's spaced Fair, it outranges a lot of things. You just have to practice the timing because it doesn't have long active frames. It's pretty useful against Mario as he suffers from lack of disjoint, but it's a staple in pretty much all of Greninja's neutral game. I think Greninja's Ftilt is underrated, as it's faster than his F smash and has almost the same reach but with no blind spot. You can also angle it up or down, which can help against different approaches. If they're landing on your shield with attacks that autocancel too fast for you to grab (Greninja's standing grab sucks), OoS options include jab and Shadow Sneak. If neither of these are working and you can't seem to do anything to punish them landing on your shield, then you're shielding in one place for too long. Crouching and waiting for a hasty approach works too, since you reduce your hurtbox and can respond with Dtilt.
Shuriken pokes are good too, especially if the other character doesn't have great speed. You can SH before sending one off if you need to hit them in the air. You can also B-reverse or Wavebounce to give yourself a bit of extra room to work with.
Players that try to create walls like this usually get themselves stuck in patterns more than other players, and you can take advantage of this as well. If they are floaty characters, try getting underneath them if it's safe. If not, you should at least be able to dash underneath them and get behind them, either to hit them with a Bair or a pivoted tilt or F smash, or to just get away and make space. Perfect shielding is much easier in Smash 4 and if the other player is throwing out a predictable wall of moves, it should be even easier to execute. Don't let the other player scare you into shielding more than you have to, IMO Greninja's #1 weakness is shield pressure.
Hydro Pump is great for resetting to neutral. Keep in mind it has a little bit of startup, but if you have the space you can just push them away. It won't do much damage but it can keep he pressure off and can mess with the other player's rhythm.
Just remember that an approach isn't made up of just moving towards your opponent and Greninja has to bait a lot. You have less options out of a dash than you do out of walking, so remember that you don't always have to go fast. Sometimes, just waiting in one place is the best option. If they're tossing out lots of quick moves waiting for you to charge in, just walk the other way and see what they do.
Hope this gives some ideas.