The reason gunk shot makes him op is that it means there is now no single type (as far as i'm aware) that can wall a greninja regardless of his set. This means he is similar to M-lucaria who could be special or physical you basically had to sack a pokemon to find out what set it is. Greninja while not being that bad is certainly close now + given his high base att mixed sets are now appearing here or there so you have to bear that in mind as well.
So its not really the move that breaks him but the potential it gives him as it adds a lot of viability onto a physical set as well as making mixed sets stronger.
As for whats best for him it now, it depends on your team you just customize greninja to fill the gap on your team and cover your weaknesses so whatever set does that best is likely to be the best otherwise if your building around him do whatever your want to do
So its not really the move that breaks him but the potential it gives him as it adds a lot of viability onto a physical set as well as making mixed sets stronger.
As for whats best for him it now, it depends on your team you just customize greninja to fill the gap on your team and cover your weaknesses so whatever set does that best is likely to be the best otherwise if your building around him do whatever your want to do
