Zweilous's Dark / Dragon typing grants it many resistances to common special attacks, and with 72 / 70 / 70 defenses alongside Eviolite, it can take on a huge slew of special attackers in the tier. The list of Pokemon that Zweilous beats is huge; these Pokemon provide it opportunities to phaze teams around with Roar and Dragon Tail to rack up damage from Stealth Rock and Spikes. Pokemon such as Eelektross, Rotom-S, Zebstrika, Serperior, Exeggutor, Ludicolo, Alomomola, Musharna, Charizard, Misdreavus, and Haunter make up a large portion of the metagame and are easily walled and taken advantage of by Zweilous.
Zweilous's main draw, other than walling special attackers, is its ability to phaze other Pokemon with both Roar and Dragon Tail. Roar ignores the effects of Hustle and allows for more reliable phazing, while Dragon Tail ignores the effects of Taunt and deals damage but only has a shaky 72% accuracy when putting Hustle into account. Having both Roar and Dragon Tail at once might seem weird, but it maximizes the chances of Sleep Talk picking a phazing move, which is extremely useful to keep a phazing chain going. Crunch is a good option over Dragon Tail for 2HKOing Musharna, Duosion, and Misdreavus, but its use ends there. Crunch gets in the way of Sleep Talk's phazing and only deals decent damage to neutral targets. However, if your team is weak to those targets, Crunch can be a blessing as the combination of good special bulk, phazing, and ability to beat Musharna, Duosion, and Misdreavus is not found on many other Pokemon in the tier.
60 Speed EVs allow Zweilous to outspeed uninvested Alomomola and Tangela while also phazing them through Sleep Talk before they can do anything in return. This is really useful for Zweilous as it prevents Alomomola and Tangela from using Wish or Leech Seed, respectively, which would decrease the damage done on the Pokemon being forced in to take damage from entry hazards. With all the talk about entry hazards, it should come to no surprise that some of the best partners for Zweilous are Stealth Rock and Spikes users. As this set is more tailored to fit on defensive teams, the best Stealth Rock users for the job are Metang, Bastiodon, Torkoal, and Regirock. Each have niches for stall teams and pair well with Zweilous, especially Metang and Torkoal, who have good defensive synergy with it and support it by countering Jynx and providing Rapid Spin support, respectively.
Rapid Spin users in general are very good for Zweilous, who doesn't like having to switch into entry hazards while attempting to wall the Pokemon it needs to. Torkoal and Wartortle are the best defensive Rapid Spin users in the tier; Torkoal is a good check to Scolipede, Primeape, and Jynx that bother Zweilous, while Wartortle can guarantee Rapid Spin thanks to Foresight. As Zweilous is constantly using Rest, Heal Bell and Aromatherapy are great support options for it. They allow Zweilous to instantly wake up without having to be out on the field for two turns, which is useful if it desperately needs to be awake to heal with Rest or phaze with Roar. Musharna and Misdreavus are the best users of Heal Bell to partner with Zweilous as they can take the Fighting-type attacks aimed at it. Meganium, Audino, and Lickilicky are also good options to use Aromatherapy or Heal Bell on stall teams, but they compound weaknesses with Zweilous.