EDIT: Shadow Sneak is major bs
it's easy to predict when you know how.
greninja walks with his hands crossed kinda when he's using it, and a little shadow appears. the shadow moves at about twice the speed he does (the shadow can't be seen on stages like Flat Zone and Duck Hunt, so be careful)
when you see greninja disappear, drop everything you're doing and spot-dodge (hold down while shielding) then punish with any move