I agree with ShakeItUp Dusknoir is really bad Pokemon to be B Rank. But, I think he shouldnt be E Rank (this should be reserved only to Munchlax), but should be D Rank. Although Dusknoir has a decent 100 Base Attack, his moves are really weak, and mostly his STAB moves. You need to invest a lot of evs on Attack if you can do any damage, and this is really bad, because Disknoir needs evs on Defenses.
Like ShakeItUp said Dusknoir needs Pain Split, Will-o-Wisp and Shadow Sneak, but I think the last move should be Seismic Toss and not any of the other randoms moves that I could use. Seismic Toss leaves it a consistent damage to a lot of stuff that laughs in his face if it is using random moves like Moltres, Rotom-M, Druddigon, Nidoqueen... And anything that isnt ghost that is hit harder by Shadow Sneak (besides Spiritomb).
Other thing is that Dusknoir can not counter Cryogonal as well as Spiritomb does. Dusknoir simply can not trap it. Furthermore, if you want to do damage on Cryogonal you need to put evs on attack an use something like Fire Punch or Drain Punch that are really bad and weak, even against a super-effective Pokemon. Another thing, its that if use SubPunch, you cannot do anything to Cryogonal lol, besides you use a lot of Evs on SDef, and this set is even worse than the Defensive by far.
Like a Weather Support Dusknoir its bad again. Dusknoir its really bad because cannot support so much besides rain and a random burn. There a lot of Bulky Pokemon that can provide better support and Rain Dance / Sunny Day as well, for name few like Uxie, Lanturn, Rotom-M, Volbeat, Regirock, Mesprit...