After testing a bit Skuntank, I found that's bad that I suppose. Skuntank its pretty mediocre doing his job: trapping Psychic and Ghost-types. Its really difficult justify the use of Skuntank over other trappers like Spiritomb or Drapion. Firstly, Skuntank hasnt anything important that make it better or different, on the contrary, Skuntank lacks important points; Skuntank unlike Drapion hasnt good speed, 84 Base Speed is really mediocre, this means its force to use the predictable Sucker Punch against anything that outsped him. About bulks difference between Drapion and Skuntank, aren't important really, Drapion take a bit better physical attacks, Skuntank take a bit better special attacks, but in general arent significally important to argumment with it. One of the most important difference between both is the movepool. Speaking of coverage, Skuntank's coverage moves are quite poor; his usable moves are Sucker Punch, Pursuit, Crunch, Fire Blast, Explosion and Poison Jab. This means that there are a lot of Pokemon that can exploitable Skuntank's coverage; anything that resist Dark-type is a great check to Skuntank like Aggron, Crawdaunt, Drapion, Poliwrath, Emboar, Escavalier, Steelix or Klinklang for name few, some of these like Drapion or Klinklang can set up without issue. Also, isnt neccesary use a Pokemon that resist Dark moves, because 93 Base Att is not that good and its main moves to hit are Sucker Punch / Pursuit, its pretty easy to check with things such as Rhydon, Qwilfish, Omastar, Miltank, Tangrowth, Regirock or Fraxure for example. In the other hand, Drapion unlike Skuntank, with Earthquake and Swords Dance, can threaten most of the aforementioned Pokemon. About Aftermath is a pretty good ability, I think is a way of stretching it; Aftermath would useful in some specific situations, but its not enough to justify the use of Skuntank.
I early found that Skuntank is not that great at doing his job; in theory can trap and kill without a lot problems Psychic and Ghost-types, but needs to be carefully when switching. Pokemon that Skuntank is suppose to defeat, like Misdreavus, Uxie, Cresselia or Rotom, use status moves, and because this metagame is plague of Dark-types, is not difficult to predict the switching, and cripple it with Will-o-Wisp / Thunder Wave that hurts a lot Skuntank. Others like Golurk or Spiritomb, put you into controversial situation; Golurk can threaten you with Earthquake, and you need to predict very well between Sucker Punch / Pursuit or even with Crunch to kill him. Spiritomb can burn you, trick you, or even hit really hard with Return; about Crotomb version, you cannot do anything, Spiritomb set up on you with relative ease, even if you have Taunt, Spiritomb can take Sucker Punch / Pursuit on the switching, witout receive 3HKO from both.
Overall, Skuntank is not that great at doing his job, and stiff a lot of competition with Drapion and Spiritomb. Also, if the opposite team lacks Ghost or Psychic-types I found that Skuntank becomes to be useless, outside revenge kill anything frail like Sceptile or Galvantula. Skuntank would be use on some specific teams, but in general terms is not great Pokemon. Low C Rank, its fine for it.