Always glad to see new people interested in the frog. Right now, there aren't any advanced techniques you really absolutely need to learn with Greninja. Instead, the first thing you need to do is become comfortable with Greninja's movement. Greninja has excellent mobility, so it's important that you adapt to it and become adept at using it as quickly as possible. The next thing, learn to recover with him. This may sound silly and patronizing, but don't be surprised if you initially find yourself Shadow Sneaking accidentally (easier to recover from now, but it's still a pain if it happens). Greninja has a fantastic recovery; you just need to take a little time to become adept with it. Finally, learn his moves, all of his moves. Learn their spacing, knockback, start-up and cool down. Greninja cannot afford to be surprised by a move that comes out faster than his own. There are very few moves Greninja can rely on in more than a couple situations (although, jab, dash grab, and Fair come to mind as some of his best moves). All of Greninja's moves work in tandem with one another, and they all have their uses (for instance, even the relatively useless Ftilt can be used as a punish if someone hits the back of your shield and their cool-down ends before Fsmash can come out).
Now, this may sound super silly, but it's of the utmost importance, and this next statement will answer your second question: Yes, Greninja is among the most difficult characters to play in this game in my opinion. Every step you take has to be calculated. Every move you make needs a reason behind it, with near certainty it will allow you to stay in your current position or elevate you to a better one. You can still take risks with Greninja (Usmash, Fsmash, etc.,), but even when you do, you need to be sure that a possible punishment from those risks is as minor as possible. Greninja is hugely momentum based (as is Smash 4 in general), so giving away momentum with a risk that goes awry can be a big deal. On the other hand, if it works, unless it's a KO, the pay-off often isn't worth it in my opinion.
After you're comfortable with Greninja, I'd advise looking into some combos (and I'm sure you'll have found some on your own by this point). There's a wonderful combo thread by MartinAW4 right here:
http://smashboards.com/threads/greninjas-true-combos.375003/
One thing you may find yourself struggling with is getting kills, but do note that Greninja does have a kill throw in Uthrow. However, this is nearly entirely reliant on rage, and as you get better with Greninja, you'll soon come to realize Greninja is
heavily rage reliant for kills and is thus a lead character. A Greninja with a stock deficit is in a terrible situation more often than not. Uthrow doesn't kill until 162% on average with no rage, and the opponent has enough knockback via their rage to get good sized combos on Greninja. On the other hand, Greninja with a lead is a
fantastic character. Lead Greninja is probably in the top 10 best characters in the game. Given his mobility and spacing tools, Greninja is extremely difficult to even grab, and on top of that, he has a fantastic punish game. Thus, you should definitely take advantage of this as you become better with the character. However, never try to force a kill as most of Greninja's kill options have a fair amount of lag on them. With rage, your best kill option is easily Uthrow. Greninja has a fantastic dash grab, and as detailed in this thread:
http://smashboards.com/threads/repo...cents-shadow-sneak-percents-added-wip.383093/ Uthrow can kill surprisingly early.
Finally, I believe Greninja to be a low high tier. The patch has made some of his bad MUs (notably Sonic and Sheik) a bit easier. MUs have been his primary struggle for quite a while, and now that his load's been lightened a bit, I do feel he could just barely be considered a high tier character.
In conclusion, Greninja's a hell of a lot of work. There's a ton to consider while playing him, but once you become really adept at using him, he really is a ton of fun to use. You could always just pick up a top tier, sure, but if you find that Greninja fits your playstyle best, I'd definitely recommend using him over top tiers that may not suit you as well. Keep practicing, and don't get discouraged if sometimes things don't work out; just think about what may have gone wrong, maybe even ask other Greninjas for advice, and then adjust your gameplan accordingly. Greninja doesn't have any overtly terrible match-ups, so a lot of the time, it really is just figuring out how your opponent is playing and figuring out ways to mitigate their strategy as I personally believe there is no one single strategy that is able to completely wall Greninja out.
