>>17105018If unpredictability is the sole reason for running Greninja, why not run a mixed or special set?
You have your choice any of the Hidden Powers, and that's really, really unpredictable. If your team is missing, say a Lightning attack, boom you have a pretty strong STAB Lightning attacker. I could go on...
>>17105178Having a nice pivoting core is very good indeed. I use two pivots myself in my main team.
>>17105218While this is true, you have to consider what priority attacks Greninja can seriously take even with a nature that doesn't impair his defense.
I run Hasty and know Greninja can tank a Choice Band Scizor Bullet Punch (or Mega Scizor unboosted Bullet Punch), Shadow Sneaks from Aegislash all day long, and unboosted Aqua Jets, but that's about it.
You do not want to be forced into a position to tank Brave Birds, Mach Punches (or Vacuum Waves for that matter), Extremespeeds, or Quick Attacks, and this is why (these are all done with a nature that doesn't hinder defense, expect them to hit even harder with Hasty):
252 Atk Choice Band Talonflame Brave Bird vs. 4 HP / 0 Def Greninja: 297-349 (103.8 - 122%) -- guaranteed OHKO
252 Atk Life Orb Technician Breloom Mach Punch vs. 4 HP / 0 Def Greninja: 354-421 (123.7 - 147.2%) -- guaranteed OHKO
252+ Atk Choice Band Dragonite Extreme Speed vs. 4 HP / 0 Def Greninja: 204-240 (71.3 - 83.9%) -- guaranteed 2HKO
252+ Atk Choice Band Huge Power Diggersby Quick Attack vs. 4 HP / 0 Def Greninja: 177-208 (61.8 - 72.7%) -- guaranteed 2HKO
So, generally if you're scout leading with Greninja and they lead with Conkeldurr or Breloom, do not stay in. Do not attempt to U-Turn. You're going to get OHKO'd and be behind the eight ball out of the gates.
If you're interested on getting an idea of what your Greninja set could tank, run simulated damage tests here:
http://pokemonshowdown.com/damagecalc/ 
