Despite the existence of Fairy type, I would actually go ahead and suggest a sort of "Cro-Garde" set.
Zygarde@Leftovers
Nature: Careful
252 HP/252 SpD/4 Atk
Rest
Sleep talk
Dragon Tail
Coil
This set sacrifices the ability to hit almost any opponent for the ability to be an annoying prick to your opponent. Certainly; it is a requirement to deal with any and all Fairy types on your opponent's team before trying to set up with this guy, but once that happens this thing can be a real pain to deal with. One of CroCune's main issues was that other Calm Minders could set up alongside it, and it was somewhat vulnerable to being phazed if it didn't have enough boosts to OHKO with Surf/Scald. With CroGarde, that is no longer an issue, as Zygarde has enough speed to outpace most Pokemon that run phasing moves. The EV spread and nature follow similar logic to the EV spread used by Cro-Cune: put as much natural bulk as you can into the defense that you can't boost.
Entry hazard support is practically essential for this sort of set to work. Good partners for this set include any Pokemon with powerful Steel or Poison moves to deal with prevalent Fairy types. Gengar receives a special mention here for its ability to trap opponents (if Mega), as does Magnezone for its ability to trap the ever-annoying Klefki and threaten Azumarill. Although it doesn't necessarily beat Fairy types on its own, Skarmory also serves as a powerful partner to Zygarde. Its ability to set up entry hazards and take physical hits that will likely be coming Zygarde's way from the likes of Mamoswine, Azumarill, Dragonite, and Garchomp are greatly appreciated.
Zygarde@Leftovers
Nature: Careful
252 HP/252 SpD/4 Atk
Rest
Sleep talk
Dragon Tail
Coil
This set sacrifices the ability to hit almost any opponent for the ability to be an annoying prick to your opponent. Certainly; it is a requirement to deal with any and all Fairy types on your opponent's team before trying to set up with this guy, but once that happens this thing can be a real pain to deal with. One of CroCune's main issues was that other Calm Minders could set up alongside it, and it was somewhat vulnerable to being phazed if it didn't have enough boosts to OHKO with Surf/Scald. With CroGarde, that is no longer an issue, as Zygarde has enough speed to outpace most Pokemon that run phasing moves. The EV spread and nature follow similar logic to the EV spread used by Cro-Cune: put as much natural bulk as you can into the defense that you can't boost.
Entry hazard support is practically essential for this sort of set to work. Good partners for this set include any Pokemon with powerful Steel or Poison moves to deal with prevalent Fairy types. Gengar receives a special mention here for its ability to trap opponents (if Mega), as does Magnezone for its ability to trap the ever-annoying Klefki and threaten Azumarill. Although it doesn't necessarily beat Fairy types on its own, Skarmory also serves as a powerful partner to Zygarde. Its ability to set up entry hazards and take physical hits that will likely be coming Zygarde's way from the likes of Mamoswine, Azumarill, Dragonite, and Garchomp are greatly appreciated.