Zygarde@Leftovers/Yache Berry
252 HP/252 Atk/4 SpD
Adamant Nature
Dragon Dance
Coil
Earthquake
Stone Edge
You're probably wondering why I included both Coil AND Dragon Dance. The thing is, Zygarde can't manage with either one alone.
If you try running just Dragon Dance to make him a sweeper, the problem is his lack of good/reliable sweeping moves.
The problems with Dragon Dance:
-Outrage is a bad move for a sweeper, because it leaves him vulnerable; your opponent can send in a Fairy or Steel type Pokemon to begin setup or countering while you waste a turn or two with an ineffective attack. And in addition, even if they don't use a fairy/steel type, the best you can hope for is confusion. Outrage works well for non-setup based Pokemon, like Garchomp, but if you intend to sweep, it's gonna betray you.
-Stone Edge is also bad for a sweeper; although powerful, its unreliable accuracy can give your opponent the opportunity to counter you if you miss.
-Return is reliable and somewhat powerful, but it does nothing for coverage, and its lack of STAB on Zygarde renders it rather ineffective.
-Extreme Speed is a great move, but Zygarde with DD doesn't have much use for it. He outspeeds everything after a few boosts, and even if they go for a priority move, there is no common priority user that poses a threat to Zygarde's great physical bulk. Yes, I already checked for Ice Shard - even a Choice-Banded 252+ Mamoswine isn't guaranteed to OHKO Zygarde after 252 HP investment.
-Earthquake and Crunch are the only good moves for sweeping that he has; they both have a decent power and 100% accuracy, with no harmful side effects, while also providing coverage.
If you try running just Coil to turn him into a physical tank with access to a reliable Stone Edge, the problem is his speed and special bulk.
The problems with Coil:
Basically, the problem with Coil boils down to, "You're not fast enough to outspeed everything, and therefore, anything with Ice Beam and a decent speed stat ends your setup immediately, and all those Defense boosts were for nothing."
You might be thinking that if you run Extreme Speed, you can outspeed them to OHKO with the help of your offensive boosts. Unfortunately, an 80 BP move with no STAB and no chance of being SE is not a very reliable OHKO move, even after a few attack boosts. Granted, Extreme Speed would still be a good move to run on a Coil Zygarde; In fact, a Coil set isn't really a bad idea - it's just an easily counterable one.
So, try running Coil AND Dragon Dance, with Earthquake/Stone Edge. Here's how it goes down:
-Earthquake/Stone Edge form a nearly unresisted type duo, with only Breloom, Torterra, Virizion, and Chesnaught resisting it. It also has great Super -Effective Coverage, being strong against a total of 363 Pokemon between the two.
-Stone Edge has its unreliable 80% accuracy fixed by Coil's Accuracy boost.
-Earthquake is an already-reliable move with 100% accuracy, 100 BP, great Super Effective coverage, and STAB.
-Dragon Dance boosts your Speed, allowing you to sweep, and outspeed those pesky Ice Beam attackers and KO them with Super Effective Stone Edge.
-Coil boosts your Defense, letting you take even less damage from priority moves. Although there's virtually no priority attacker that poses a threat to Zygarde on its own, a few of them, especially Ice Shard, would still wear him down, or KO him if he was already weakened during setup.
Between Coil and Dragon Dance, you end up with a great Attack stat in addition to a reliably powerful EdgeQuake combo, the Speed necessary to sweep, and the physical bulk to easily take priority attacks.
With priority no longer an issue due to his immense physical bulk after all those Coils, the only thing that can hope to counter Zygarde would be something bulky enough to take a hit and counter with a special-based Dragon- or Ice-type move. Well, that's all I can see anyway - I'd love to hear anyone else's thoughts on this set.