Role: Stall breaker
Format: Nat Dex AG
Gengar-Mega @ Gengarite
Ability: Cursed Body
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Nasty Plot
- Encore
- Shadow Ball
- Sludge Wave
NOTE: Nasty Plot and Encore were given to Gengar in Generation 8. Since Mega Evolution doesn't exist in Gen 8, it's only possible in Nat Dex AG formats.
The combination of Shadow Tag + Encore + Nasty Plot truly solidifies Gengar as an absurd stall abuser. It can trap on Defoggers, and more passive Pokemon like Chansey, Giratina, Lugia, etc.
Encore allows you to trap support Pokemon if they click a move such as Defog, Recover, Toxic (which is Gengar immune to), Substitute, etc. After clicking Encore, start using Nasty Plot. Shadow Ball is STAB, Sludge Wave over Sludge Bomb because of slightly higher BP, and since Gengar is so frail it won't get a chance to survive.
There are certain drawbacks now, especially after DLC 2. The most prominent of them being Calyrex-Shadow. It is able to outspeed you always, whether it's Timid or Modest. Therefore, you need a solid Calyrex-Shadow counter. Arceus-Dark, Yveltal and Tyranitar are good picks. The latter among them is able to pursuit trap it, while the former two get reliable recovery. Pairing it with a Necrozma-Dusk Mane is nice as it would allow you to handle Zacian-Crowned.
Sticky Web support is invaluable as well. But, beware of Dynamax Yveltal. Even if you were to Encore trap it into something like Sucker Punch, Dynamax does it exist. Dynamax ignores the lock of Encore, so you had be careful.
Mega-Gengar mainly fits into Balance and passive teams that have a major weakness to stall. In offense, you had generally liked Mega-Rayquaza over it. This is because it can boost its speed as well from Dragon Dance, making it ludicrously dangerous. Dynamax sweepers like Xerneas, Yveltal, etc. appreciate your ability to Encore trap opponents, generally forcing opponent to Dynamax.