3 words
Don't Even Try.
Well against the bulky ones that is
Using the sets and ghost Pokemon fondant mentioned
- 252 SpAtk Life Orb Mienshao (+SpAtk) Hidden Power Dark vs 252 HP/4
SpDef Eviolite Dusclops: 33.1% - 39.44% (3-4 hits to KO)
- 252 SpAtk Life Orb Mienshao (+SpAtk) Hidden Power Dark vs 252 HP/136
SpDef Sableye (+SpDef) : 31.25% - 36.84% [4 hits to KO (with
Leftovers)]
- 252 SpAtk Life Orb Mienshao (+SpAtk) Hidden Power Dark vs 248 HP/8
SpDef Cofagrigus: 52.98% - 62.7% [2-3 hits to KO (with Leftovers)]
- 252 SpAtk Life Orb Mienshao (+SpAtk) Hidden Power Dark vs 252 HP/4
SpDef Cofagrigus: 52.81% - 62.5% [2-3 hits to KO (with Leftovers)]
Those are the bulky ones, and you have no chance of 2HKOing Sableye or Dusclops, even with max investment. Cofagrigus is possible, but also possibly not factoring in leftovers recovery.
Against the less bulkier ones that were also mentioned;
- 0 SpAtk Life Orb Mienshao Hidden Power Dark vs 0 HP/0 SpDef
Chandelure: 52.87% - 62.84% (2 hits to KO)
- 0 SpAtk Life Orb Mienshao Hidden Power Dark vs 4 HP/0 SpDef Rotom:
64.46% - 76.45% (2 hits to KO)
- 68 SpAtk Life Orb Mienshao Hidden Power Dark vs 0 HP/0 SpDef
Mismagius: 50.96% - 59.77% (2 hits to KO)
So running 68 EVs lets you take down Mismagius, as well as Chandy and Rotom.