Hmm... big problem. HP Water is weakened by Sunny Day (a needed skill for Shedinja to avoid weather inducers, this is a shame, I wanted to nail Rock/Steel guys too...).
HP Grass it is. There, the guesses for what to do IV-wise are easy enough.
EDIT: Whoa, never mind. This guy learns SolarBeam? The Hidden Power is basically irrelevant then. Grass/Water it is to take care of Rocks early on (should I try for a HP at all?).