Okay, so Greninja keeps changing it's type so you can never hit it super effectively, and, more often than not, hits you super effectively. Annoying, isn't it? But, there is one type that will save you, and that type is electric!
The only weakness electric has is ground (durp) and the only ground move Greninja gets is DIG. Only idiots and morons use two turn attack moves because the opponent can switch freely in to something that resists it (no offense was meant if you are an idiot or a moron. sorry.) So you are safe with electric type.
Now you need one that's faster than a Greninja, so you can hit it before it changes it's type. And that's where this little beastling comes into play:
Just a quick example of an electric type that's faster than a Greninja. Just hit with a STAB thunderbolt and he's down.
Or you could get a slow, bulky electric type with lots of coverage moves, and try to predict what move he is going use, then slap him with that super effective attack! Boom!
Hope this helped!