>>41763995>252 SpA Choice Specs Hydreigon Flash Cannon vs. 252 HP / 0 SpD Sylveon: 204-240 (51.7 - 60.9%) -- 94.1% chance to 2HKO after Leftovers recoveryIt can force a 2HKO with Specs, force a 3HKO with Life Orb, and has a good chance to 3HKO with Scarf. Meanwhile with Nasty Plot sets, a +2 is always a 2HKO and with Life Orb a good chance to OHKO
>+2 252 SpA Life Orb Hydreigon Flash Cannon vs. 252 HP / 0 SpD Sylveon: 354-416 (89.8 - 105.5%) -- 37.5% chance to OHKO after Leftovers recovery. So yeah, in 1v1 Sylveon still technically wins, but Flash Cannon can make Sylveon a check at best in a 6v6, especially if Sylveon doesn't use Wish.
