Welcom to the Village Hidden in the Pond, enjoy your stay ◥ θ┴θ◤ First off...
The notable Matchups that Greninja really shines in is:
The notables ones he struggle with (in order) are:
The notable matchups that he finds tricky but still even is:
As
Jaguar360
stated, having good movement is key. His mobility is very impressive, and being able to incorporate foxtrotting, sliding pivots, perfect pivots isvital to being good with him. I recommend that you play a whole day on smash ladder WITHOUT your shield button. Greninja's OOS punish are very lacking, so breaking that habit will be important. Doing this will help you learn to just run away and weave out of attacks than just shielding. I did this and noticed immediate results.
Also, go into training mode and lab out some Nair combos. The combos vary depending on percent, but there is always a followup. You should definitely use this to punish spotdodges, cross up shields, and as a mixup when your opponent starts to read your dash grabs.
When your trying to get off a fast punish, jab and down tilt are your best bet.
Learn how to gimp people hydro pump gimp. and kill really help you net early kills. The easiest way to do this is by, when your opponent is sent flying very far vertically, jump off and position your self between them and the ledge, then hydro pump them. The best angle is to it diagonally away from your opponent.
Here are some characters it works on: