Not my set at all. Zygarde typically is walled by Reuniculus setting up Acid Armour and outboosting Zygarde, and from there can set up CMs and sweep. However, Dragon Tail turns this on its head and allows Zygarde to set up to +1 or +2 then phase Reuniculus out (though don't get too greedy as Reuniculus 2HKOs with Psychic at +1) and take it out when Reuniculus comes back without boosts.
Zygarde @ Groundium Z/Figy Berry/Leftovers/Earth Plate/Weakness Policy/Dragonium Z/Chesto Berry
Ability: Aura Break
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature
- Dragon Dance
- Thousand Arrows
-
Dragon Tail
- Extreme Speed/Substitute/Glare/Outrage/Iron Tail/Coil/Rest/Whatever you like
+1 Dragon Tail (hitting Reuniculus at up to +2) to Tectonic Rage KOs 99% of the time, but if you want to save your Z-Crystal then you can use one of the many other items Zygarde can run. The fourth move is filler and depends on what your team needs.
The large amount of slashes are because it essentially only needs D-Dance, Thousand Arrows, and D-Tail, and the item is somewhat irrelevant. But be sensible, if you're running sub without a Z-Move, then lefties or a berry are pretty nice.
This set also works well against other setup sweepers, most obviously Curse Mega-Scizor, but also Stored Power Mega-Latias, and again shows the crazy variety Zygarde can have just because of how good Thousand arrows is as coverage, its good stats, and the variety of setup moves Zygarde gets.
Here's a replay from OLT from which I took the mon from (not my own; if there is an etiquette thing about not reposting other people's replays let me know and I'll delete it):
https://replay.pokemonshowdown.com/gen7ou-780617175