Greninja has the Hidden Ability "Protean."
"Changes the Pokémon's type to the type of the move it just used." This gives the indication it changes type after it uses the move...

...but that's NOT TRUE. It changes type BEFORE Greninja uses the move. Like this:

>Go, Greninja!
>Greninja's biology is changing!
>Greninja used Shadow Sneak!
>It's Super Effective!

Every single move you put on a Protean Greninja has STAB. It will change into that type BEFORE it uses the move. Knowing this, you can:

1) STAB everything.
2) Block Spinners
3) Make him Flying-type to avoid EQs
4) Think a T-bolt is coming your way? Earth Power, now you're immune.
5) Oh no I'm so scared of that Fighting type I'm facing, he's gonna Hi-Jump Kick me oops I used Extrasensory It's Super-Effective against him and now I took half damage from the attack lol

Bretty Gud, yeah?