>>292437633hit water specs shuriken A-Greninja
252 SpA Choice Specs Greninja Water Shuriken (3 hits) vs. 252 HP / 0 SpD Mew: 171-201 (42.3 - 49.7%) -- guaranteed 3HKO
Meh. suicide orb works better with him because he have such a nice moveset.
Honestly, the best set will be probably something like that -
Ice beam (pretty much the best way to go Greninjash is killing something weak to ice)
Dark Pulse (Second best option)
Water Shuriken (finishing water-weak only for Getting A-Greninja, then it's a gamble on netural hits, but can be pretty much the strongest netural move)
Low Kick/Gunk Shot (Can kill stuff easy enough too)
Hidden Power Fire will be really meh to even try.