I think it's Ferrothorn but as it's taken I'll do Klefki
Klefki @ Leftovers
Ability: Prankster
EVs: 252 HP / 4 Def / 252 SpD
Calm Nature
- Flash Cannon
- Spikes
- Thunder Wave
- Toxic / Magnet Rise
Klefki does a nice job of dealing with Mega Diancie. Due to the team not really fearing Mega Sableye or Garchomp or anything of the like I didn't feel like Fairy-type coverage was particularly necessary, and Klefki doesn't really fear TankChomp regardless of it's choice of fourth move because Toxic does a good job of wearing it down whereas Magnet Rise means that it's only way of damaging you is Fire Blast, which you can p. consistently lay spikes on due to uninvested, itemless, negative-natured Fire Blast is weak. I think Magnet Rise is pretty poor on Klefki nowadays anyway as Toxic achieves most of what it does and Earth Power Diancie isn't good relative to HP Fire given that Heatran gets 2HKOed by max attack Diamond Storm anyway, but said move has some merit on this team specifically for Earth Power Diancie regardless of how rare it is. In addition to it fucking with Diancie between Flash Cannon and a potential Magnet Rise, Klefki does a really nice job as a spiker, being able to lay them repeatedly and reliably due to Prankster.