Moves
Calm Mind boosts Dusclops's Special Attack and Special Defense, granting it a way to sweep. Rest is a mandatory source of recovery because, without any means of recovering health, Dusclops would get worn down too fast. Will-O-Wisp is the best option in the third moveslot, as it cripples physical attackers, making it even easier for Dusclops to set up on them, and it is the only move that has any use before attempting to sweep. Will-O-Wisp is especially useful against Dark-types, such as Mightyena and Pawniard, that tend to switch in against Dusclops. Shadow Ball can also be used, as a STAB move with solid power that hits opposing Ghost-types that like to switch in against Dusclops and Taunt it, such as Misdreavus. However, Shadow Ball should only be used if you're using Ice Beam in the fourth moveslot, as it has little synergy with Dark Pulse. Sleep Talk is another option here, as it ensures that Dusclops does not just sit there after using Rest and makes Dusclops able to set up on Tangela and Jumpluff. Ice Beam has good super effective coverage and hits Pokemon immune or resistant to Shadow Ball if you decide to use it. Dark Pulse is an alternative option as it also hits Ghost-types and has decent neutral offensive coverage. Shadow Ball mostly hits the same targets but forces Dusclops to forgo Will-O-Wisp and cannot deal any damage against Normal-types.
Set Details
The given EV spread allows Dusclops to easily take most physical hits while boosting its Special Defense and Special Attack with Calm Mind. Dusclops can afford Special Attack investment, as it already has more than enough bulk. Pressure is the ability of choice because it is the only chance for Dusclops to win PP wars. Eviolite gives a 1.5x boost to both of its defenses and is the only thing that keeps Dusclops from being totally outclassed by Dusknoir.
Usage Tips
Set up mainly against Pokemon like Bouffalant, Arbok, and Swanna that fail to break through Dusclops's bulk. Do not try to set up with Dusclops if the opponent still has a threatening setup sweeper alive and you don't have a switch-in for it, as Dusclops is a very passive Pokemon and multiple threats to defensive teams, such as Ninetales, Duosion, and Ursaring, are able to set up on it. Double switches are very important in situations like these. Dusclops works best late-game when most setup sweepers and Taunt users, such as Misdreavus and Simipour, which are extremely annoying for Dusclops, are gone, as it heavily relies on Calm Mind and Rest. However, it can also be used earlier in the game, even when you're not attempting to set up, to counter Pokemon, such as Jumpluff and Tangela, as long as you're not too generous with switching Dusclops in against Pokemon that might carry Knock Off.
Team Options
Knock Off absorbers such as Muk can help to protect Dusclops's Eviolite. Stealth Rock, Spikes, and Toxic Spikes support from Golem, Quilladin, and Roselia among others helps wear down the opposing team. Toxic Spikes in particular is convenient because it makes Dusclops capable of outstalling Pokemon such as Audino that would otherwise be able to stop it. Dusclops can also block Rapid Spin, which is nice for entry hazard stacking teams. However, most, if not all, Defog users can remove entry hazards against it. Dusclops should only be used on defensive teams, so other walls such as Audino and Altaria are mandatory teammates.