So it appears Dedenne's only Fairy-type move is Play Rough. It's a special attacker and yet Play Rough is a physical move. The design makes zero sense, unless GF was intentionally making this Pokemon bad; in that case, it worked. And if you wanted to OHKO Zygarde, any Ice move will suffice. You don't really need SE on Dragons seeing as Fairy takes no damage from Dragon in the first place. Use them as setup fodder or a free switch-in to a predictable coverage move.
Also, the general rule of thumb is to only have one STAB move on a Pokemon per type. So stick with just Hydro Pump for Blastoise. I'd actually recommend Surf over it since HP has 80% accuracy, but whatever floats your boat.