Don't use shadow punch, since Dusknoir has mediocre attack. Go for Night Shade.
I'll bandy around two interesting sets I've used.
Calmnoir
Nature: Bold
252 special attack/252 defense/4 special defense
Item:Leftovers
Calm Mind
Pain Split
Shadow Ball
Ice Beam/Thunderbolt/Hidden Power Fighting/Dark Pulse/Focus Blast
This is a pretty awesome getup. Once you start Calm Minding, you become a fighting force as an unlikely sweeper with a move that will restore you to full health, and dent the opponent as well. Shadow Ball is your default sweeping move, though hp fighting or focus blast is nice to have something against dark types. Ice Beam or Thunderbolt really only warrant using if you're desperate for type coverage. It needs some time to get started, but once it does you can take and dish out heavy punishment.
Pseudohazer
Nature: Impish
Item: Leftovers
252 hp/120 defense/136 special defense
Mean Look
Confuse Ray
Destiny Bond
Night Shade
This is something of an unusual Dusknoir that's meant to get a single ko to force a switch and exit to stage right. The idea is that if your opponent has been statbuffing, or proving all around problematic (think of, say, a nasty choice scarfer), you keep using Destiny Bond to force a switch. If they refuse, they lose a pokemon. If they do, whoever comes in eats a confuse ray. From here on in, you can safely use Mean Look to trap them, and either deplete their health with Night Shade or let them knock you out, feinting them with Destiny Bond.
Enjoy.
