>>33496331I already shown Corkscrew Crash. There's the potential to KO Sylveon with it, but you need to get some prior damage on Sylveon first.
Life Orb:
252 SpA Life Orb Hydreigon Flash Cannon vs. 252 HP / 4 SpD Sylveon: 177-208 (44.9 - 52.7%) -- 84.4% chance to 2HKO after Stealth Rock and Leftovers recovery
252+ SpA Life Orb Hydreigon Flash Cannon vs. 252 HP / 4 SpD Sylveon: 192-229 (48.7 - 58.1%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Life Orb Hydreigon Iron Tail vs. 252 HP / 252+ Def Sylveon: 231-273 (58.6 - 69.2%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252+ Atk Life Orb Hydreigon Iron Tail vs. 252 HP / 252+ Def Sylveon: 252-299 (63.9 - 75.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Choice Spec:
252 SpA Choice Specs Hydreigon Flash Cannon vs. 252 HP / 4 SpD Sylveon: 204-240 (51.7 - 60.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252+ SpA Choice Specs Hydreigon Flash Cannon vs. 252 HP / 4 SpD Sylveon: 222-262 (56.3 - 66.4%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery