>>30080322252+ SpA Magearna Fleur Cannon vs. 252 HP / 252+ SpD Zygarde: 348-410 (56.1 - 66.1%) -- guaranteed 2HKO after Leftovers recovery
Second hit won't KO at -2 either.
252+ SpA Pheromosa Ice Beam vs. 252 HP / 252+ SpD Zygarde: 336-396 (54.1 - 63.8%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Protean Greninja Ice Beam vs. 252 HP / 252+ SpD Zygarde: 408-484 (65.8 - 78%) -- guaranteed 2HKO after Leftovers recovery
a-ninetails can sometimes oneshot with blizzard though
and yes 252+ SpD is the standard Zygarde set
If you're that concerned about getting 1HKOed as 50%, you can run sashed 10% too