Dusknoir is really fucking awful, and should be moved to at least E(it is that bad). The only reason to use it over Spiritomb that does the spin blocker thing better or Golurk for simply any offensive set, is because randoms think OMG 135 DEFENSES MUST BE THE SECOND COMING OF JESUS. And while 135 defenses are pretty good, a pathetic HP stat of 45 is not helping Dusknoir at all, allowing anything decently offensive to simply 2hko it with minimal hazard support(read: just SR most of time). Also, because its damage output is so terrible, it is set up bait for a lot of dangerous mons such as Fraxure, Liligant, Klinklang if lacking earthquake, and even if it has that, Moltres gets easily a sub against it that is 2hkoed at best by shadow sneak that is more than enough turns for moltres to get the 2hko with Fire Blast or Hurricane. Using seismic toss on anything that has a decent attack is just pathetic and a terrible excuse to justify it being higher on this viability list. It also loses to the mighty metang 1 vs 1, lol.
Random calcs of stuff 2hkoing Dusknoir with just sr that are realy common/good/decently strong. All of this are with both the most used spread by the mighty and insanely good ladder and the current analysis spread: