Well, MHera is more of a check. Unless Greninja has HP flying.
I just mentioned it because it can deal with a focus sash. --- You can't call him out for being wrong when he never intended to be right - Stephen Colbert "Bad Mojo" is not a diagnosis - Dr. Foreman, House, M.D.
If he leads with it, use something with Fake Out, and have it hold a Focus Sash as well. You will get the first hit, and then finish off Greninja with another attack. This will beat your friend's Greninja every time
You use Fake Out, he flinches, then he will go first (if it is Timid and not Modest) and bring you down to 1 HP. Your second turn should be Persian using Flail(200 BP STAB attack), wiping out Greninja, and possibly his next Pokemon. --- FC- 3797 7330 8352 IGN- Sha-Quanda
Aside from dedicated special walls, here's the best that Greninja can do to a zero SpDef invested Milotic 252 SpA Protean Greninja Extrasensory/Dark Pulse (same BP) vs. 252 HP / 0 SpD Milotic: 93-109 (23.6 - 27.6%) -- possible 5HKO after Leftovers recovery
Just Toxic it and recover.
Gastrodon works similarly with the same set, but has a chance at being 4HKOd
Also, don't take this as a counter to ALL Greninjas, one's with Grass Know (like mine) will ruin your day. --- 3DS FC: 5198-2590-6008 Patrolling the Pokemon X/Y message boards almost makes you wish for a nuclear winter...