My post from the pit thread, mildly modified:
A couple of greninja's weaknesses-
1. If he's in the air, he actually has very few options for attacking below him. Dair isn't awful, but it's EXTREMELY punishable. He can cancel the landing lag with side B, but that's also got a year and a half of lag on the end, so if he's above you there's very little to fear as long as you're ready for dair.
2. Nair has absurd ending lag if he doesn't land. Watch for it if he does it high up or over the edge(he shouldn't, but just be aware).
3. He has almost 0 true combos. Almost all of his damage comes from good reads and poking away with projectiles. Fair also has significant startup, although it's not obvious unless you play him.
Beyond that there isn't much to say. He's got very good recovery, especially on levels with walls, but that doesn't mean you can't go out there and gimp him. His power is that he's got very few weakness or strengths.
Pump does seem very powerfuls, so maybe use side B to recover, since if the pump kicks you up you can recover with up B still. Other than that you just need to play solid. Watch out for his tilts and kill moves(full charged B, pump gimps, fair, uair, smashes(up is his best by FAR) side B, and at high %'s bair).
Removed the pit specific stuff. A lot of the same info still applies. Greninja is just solid all around mostly, but also forces solid play to see results, so if you actually run into a good one you need to have your fundamentals down as well.
The only thing i'd add is watch for his laggy moves(side B is AWFUL most times) and punish accordingly. Dropping Koopa's can disrupt his game, but you can't just spam them since I think B just blows them up.
