Gengar, Froslass, or Giratina @ Shed Shell, Leftovers, or Lum Berry
Ability: Normalize
Skill Swap
Mean Look
Taunt/Encore
Perish Song
The choice of Pokemon depends on what you're doing. Gengar is the posterboy for the Normalize-Skill-Swap set, so you can pop him in and prey on your opponent expecting an immediate Skill Swap only for them to get trapped by Mean Look. Froslass is an option over Gengar as it has the same speed, slightly higher bulk, and, most importantly, immunity to residual damage from Hail, which matters if you're running a Hail team as there's no room to pack in recovery (as such, I recommend having Wish somewhere if you use this set). Giratina is, by far, the bulkiest option, but it's slower and tends to lure in Magic Bouncers which can be problematic if you use the wrong move.
You can likely expect how the set works just from the abilities. Trap an unwitting victim with Mean Look (because of Magic Bounce, this makes Shed Shell the preferred item). Then Skill Swap em if they're an attacker (or have Volturn) or Taunt if they're support. Once they're disabled, Perish Song. Normalize ensures your Ghost cannot be KOed by attacks unless they're packing Foresight, which gets stopped by Taunt or Encore. Taunt/Encore ensure that you can't be phazed, set-up upon, or that they can't Baton Pass to freedom.
Of course, this set is likely only going to work once as intended per battle. However, Perish Trapping the right victim can ensure victory regardless. And even then, the movepool is so disruptive that it's mere presence can make your opponent think twice about what they're doing when it switches in. Taunt shuts down support, Skill Swap shuts down Pokemon reliant on their abilities to do their job, like Poison Heal Gigas or most Magic Bouncers while also effectively ending any offensive momentum, and the constant threat of getting Perish Trapped can hinder an opponent's ability to set-up.
If you're curious, I was looking at the Normalize Gengar set and thinking "You know, there has to be a better way to leverage this gimmick." About ten minutes later, this set was born and it's been quite effective when I've used it.