Gengar @ Gengarite
Ability: Levitate
EVs: 188 HP / 116 Def / 4 SpA / 4 SpD / 196 Spe
Timid Nature
- Destiny Bond
- Hypnosis
- Hex
- Focus Blast
Now that we have a full core selected, it's worth considering a backup mega.
Casino Gengar has been my go-to for that role recently. In addition to the "delete target Pokemon with Destiny Bond" utility that comes built-in with Mega Gengar, this build has the ability to, with some luck, cripple and remove multiple members of the opponent's team.
Hypnosis is great in general with this fast and bulky build, but it's especially effective when trying to avoid Sucker Punch mind games and on predicted switches the turn that Gengar mega-evolves.
Focus Blast is primarily for the Tyranitars that stall teams almost always bring against Gengar, but it's also useful against Kangaskhan, Porygon2, Ferrothorn, and more. (Missing doesn't hurt as much when they're asleep!)
No matter how well you craft your team, sometimes your opponent has one or more mons that you can't deal with effectively or their team just generally matches up well against yours. In these cases, coin flipping your way to victory can provide a better chance of winning than anything else you can do. Casino Gengar provides that option.