Honestly, I really disagree with Zweilous even being C rank, let alone B Rank.
Zweilous is slow as molasses, and it's Hustle Boosted attacks all suffer from accuracy issues. Even with Evolite it fails to avoid being 2HKO'ed by many of NU's top threats, especially since a lot of them pack Fighting-type moves [Braviery, Zangoose, Emboar and Golurk all tend to pack Base 120+ fighting attacks. Samuwott usually packs Megahorn too]
Yes, it *can* ruin something. However to do so it has to:
Get in safely and avoid being 2HKO'ed or crippled in some other way
The enemy doesn't switch in something that can withstand it's Outrage like Golem, or Metang
Actually hit. We all know Stone Edge is known for hitting!
Meanwhile he has no way to make up for the damage he sustains in those 2 turns before he can do anything [1 if revenge], nor hazard damage, because he has to run Evolite or he gets butchered by everything.
Even if Zweilous gets a KO, it's locked into Outrage, and gets promptly revenge killed anyway.
Most of the time to accomplish anything Zweilous needs something on your team to be KO'ed, and at best it'l take 1 thing down on the enemy team.
It's a slightly different matter if you are running Zweilous in a Trick Room team, but even then there's far better mons you could use that don't have to worry about being weak to the biggest threats in the tier, and accuracy, such as Marowak. Not to mention if you're running Zweilous in Trick Room that means you're giving it a buttload of team support.
It *can* work. But certainly isn't reliable to useful enough to fit the B Criteria.
B rank: Reserved for Pokemon who are great in the NU metagame. These Pokemon have more notable flaws than of those above it that affects how they function in the tier. Their positive traits still outshine their negatives, but they require a bit more team support to bring out their full potential.
It is a long streach to call Zweilous great, and it's ability to OHKO most things doesn't outshine accuracy issues, and problems getting in the first place.
C rank: Reserved for Pokemon that have notable niches in the NU metagame, but have just as notable flaws that prevent them from being effective. Pokemon in the C tier often require significant support to be effective in NU. C rank Pokemon tend to face a lot of competition with the more commonly used Pokemon.
Zweilous is here at best. On Trick Room, as I mentioned before, he faces stiff competition from things like Marowak. Outside of Trick Room he is either dead weight or at best will trade 2 for 1 [Coming in to revenge kill something and then being KO'ed]
D rank: Reserved for Pokemon that are mediocre in the NU metagame, but are decent enough to justify their use on some teams. These Pokemon are usable but have no real niche that gives them a chance to thrive in the metagame.
I would also justify D Rank, because outside of Trick Room I honestly can't think of why I would want to run a slow, frail, weakness-ridden inaccurate pokemon that requires a sacrifice to do anything.