I'd probably use pain split>sub on that dusknoir. Also, +def or +sdef nature (probably def), -satk. You can then divert some EVs into atk, and you end up with the same atk, but a higher s/def.
Anyway, I may as well post something. A possible Choice Specs set:
Gengar @ Choice Specs
Modest/Timid nature (+SAtk/Spd, -Atk)
EVs: 6 HP / 252 SAtk / 252 Spd
- Focus Blast
- Hidden Power [Ice]
- Hypnosis / filler
- Shadow Ball
Maybe divert a few speed EVs into hp or the defenses, since it doesn't need max spd. And for the record, yes, hypnosis does work with choice items. It's a similar concept as Twave on a CBGyarados, although it requires a little bit more prediction, and gives a better setup for other pokes. Of course there's always the old standby mcicegar:
Gengar @ Leftovers
Lonely/Hasty nature (+Atk/Spd, -Def)
EVs: 64 HP / 252 Atk / 188 Spd
-Focus Punch
- Hidden Power [Ice]
- Substitute / Sludge Bomb
- Thunderbolt
Those EVs there are for the Lonely version. Move 4-12 EVs from Spd to HP for the Hasty varient.