Klefki can get bodied pretty hard by Gliscor.... Thunder Wave / Toxic can't touch it. And 90% of Klefkis don't run flash cannon. And Gliscors Earthquake is typically a 2hko (maximum). So Spikes Klefki, T Wave Klefkis, Toxic Klefkis, get bopped for free. Swagger Klefki is 50/50 and I have a work around double screens klefki. But being unprepared for it kind of sucks but its not that great a Pokemon but at the same time might be the early XY meta.
Checks: Gliscor, Marvel Scale Dragonite, Electivire, Magnezone, Arcanine, Mega Houndoom, Excadrill, Hippowdon, Flygon, Garchomp, Krookodile, Alakazam, (Disable) Gengar.
Number 1 Klefki Counter: Anything with the magic bounce ability. Espeon or Xatu with magic bounce... Or ideally mega Absol. Although a +2 Foul Play after the swagger. Will still hurt glass jaw Absol.