>sylveon gets outsped and nuked by garchomp with Iron Head or Poison Jab
>goodra outspeeds and outbalks sylveon; learns sludge wave, sludge bomb, and poison tail
>salamence outspeeds and learns iron tail, which is risky but will OHKO if it hits (and it usually will)
>dragonite outspeeds and can learn iron tail; will get Iron Head when Pokebank opens up
So is there any point to using Sylveon as a dragon killer? The only psuedo legend Dragon it counters decently is Hydreigon, which is a double weakness to begin with... And even that outspeeds and will receive iron tail when Pokebank opens.