Zygarde @ Leftovers
Ability: Power Construct
EVs: 252 HP / 208 Def / 48 Spe
Impish Nature
- Glare
- Haze
- Thousand Arrows
- Rest
As Zacian usually doesn't have Play Rough as a move, defensive Zygarde is a solid switch-in:
+1 252+ Atk Zacian-Crowned Behemoth Blade vs. 252 HP / 208+ Def Zygarde: 214-253 (50.9 - 60.2%) -- 89.1% chance to 2HKO
Haze lets Zygarde comfortably live Zacian's hits; Power Construct lets it become too tanky for non-Swords Dance Zacians to overwhelm:
+1 252+ Atk Zacian-Crowned Behemoth Blade vs. 252 HP / 208+ Def Zygarde-Complete: 214-253 (33.6 - 39.7%) -- 27% chance to 3HKO after Leftovers recovery
Glare, in conjunction with the 48 Speed EVs, allows Zygarde to outspeed Jolly Zacian after paralysis. Thousand Arrows is to chip opposing Pokemon. Zygarde should be used with a Necrozma-DM, to check Zacian in different situations; Necrozma can check Zacian when there are no hazards, and Zygarde can switch in when there are Stealth Rocks/Spikes on the user's side, as then Power Construct always activates.