It could go either way depending on the set Decidueye is running.
However, it is overall more efficient to use Shadow Ball from the start, or even better, switch out to a Pokemon that combats Decidueye better. You are giving your opponent free turns to work with by faking them out, which they could, and most likely will, take advantage of (probably by killing Primarina or setting up a Swords Dance or Substitute). Meanwhile, you are doing fairly mediocre damage to Decidueye while you could be doing something much more useful.