Dusknoir @ Leftovers
Ability: Pressure
EVs: 252 HP / 248 Def / 8 SpD
Impish Nature
- Will-O-Wisp
- Pain Split
- Fire Punch / Shadow Punch
- Earthquake
This is probably the best set for Dusknoir but if you're looking for a defensive ghost type, I'd advise you to use Gourgeist over Dusknoir, Gourgeist is not only a Ghost type but a Grass type giving it a helpful water resistance and also granting it sleep immunity. Dusknoir's most powerful STAB is Shadow Punch which is only 60 BP while Gourgeist has access to Seed Bomb (80 BP) and Phantom Force (90 BP) and with Phantom Force can stall out the opponent for more Will o Wisp/Leech Seed damage, something Dusknoir couldn't possibly do in it's wildest dreams, so don't use Dusknoir, it's overall outclassed by Gourgeist.
Gourgeist-Super @ Leftovers
Ability: Frisk
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Seed Bomb
- Will-O-Wisp
- Leech Seed
- Pain Split/Phantom Force/Protect