Gengar @ Black Sludge
Ability: Cursed Body
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Hex
- Will-O-Wisp
- Taunt
- Protect / Corrosive Gas / Mean Look / Sludge Bomb / Trick
Timid max speed is the only way to go. You outspeed Adamant Weavile and Jolly Kartana, both of which normal beat Gengar, but the former may not want to risk Gengar's set (if it's unknown - i.e. could be scarfed, Focus Blast, depending on the Weavile set), at the very least you can get a burn off; the latter has an 87.5% chance of dying to unboosted Hex. Max SpAtk because why not; Gengar barely lives anything anyways and you might as well spend your EVs where the math favors them.
Hex is a very powerful Ghost move and Gengar can get multiple kills on a team in the late game if enough members are statused. This condition is aided if you have teammates that also can apply status. Will-o-Wisp is the status of choice here because it punishes the many physical attackers that want to come in on Gengar (Weavile, Hippowdon, Bisharp, etc.). Toxic is also an option, but it doesn't affect the many Steels nor Toxapex.
Taunt is necessary for Blissey and Heatran, to a lesser extent. Finally, the last slot has many options. Protect allows you to scout choiced mons, particularly Rillaboom, who may try to come in and feign Grassy Glide or Knock Off, but instead U-turns. It also gives you extra recovery, which could be the difference between living a hit or otherwise (see, again, Rillaboom: 252+ Atk Choice Band Rillaboom Grassy Glide vs. 0 HP / 4 Def Gengar in Grassy Terrain: 178-210 (68.1 - 80.4%) ).
Corrosive Gas is an interesting option to remove items and, while Knock Off is also an option, Gengar won't really be doing any damage with it, but Rocky Helmet chip would be annoying. Plus, how many mons even get to run that move? Of course, Trick is also an option, which allows you to pass Blissey your Black Sludge, gimping its long-term viability and recovery. It also might allow you to pick up other useful items, like Boots against a hazard-heavy team, or trade away a scarf that Fini tricked onto you.
Sludge Bomb is also a decent option, giving you a more immediately powerful STAB with the chance to lay down another status. It does leave much to be desired against Tran, Pex, etc., though. Finally, Mean Look is a niche option specifically meant for Blissey. In conjunction with Taunt, you can slowly PP stall and whittle down the Blissey while it can't touch you. It also means that Pex can't switch out and avoid a 2HKO when burned.