Pheromosa @ Choice Band
Ability: Beast Boost
EVs: 4 HP / 252 Atk / 252 Spe
Adamant / Jolly Nature
- Triple Axel
- Close Combat
- Lunge / U-Turn
- Drill Run
Pheromosa gets some new moves in the Crown Tundra. Triple Axel is strong Ice-type coverage for Dragons and Landorus-T. Close Combat is awesome STAB, 120 BP. No need to worry about the defense drops because Pheromosa will be outspeeding everything. Lunge is Bug-type STAB, or you can use U-Turn to pivot. Drill Run is great coverage; it has a high crit chance, too. (Of course Pheromosa is banned from OU now but....)
Now for the nature: If you want an attack boost from Beast Boost when you KO a Pokémon, use Adamant. This will make her Attack stat 410. With a Jolly nature, her Speed will be 441. Personally I think the Speed boosts become redundant after a while, especially since it's already outspeeding most Pokémon. But there's always a choice.