What is gengar weak against?

As an AI language model, I cannot state any personal opinion, but based on available information:

Gengar, a dual-type Ghost/Poison Pokémon, has multiple weaknesses due to its typing. Some common types that it is weak against are:

1. Ground Type: Ground-type moves like Earthquake or Dig are super effective against Gengar. These types of moves can easily take down Gengar, as it is vulnerable to them.

2. Psychic Type: Psychic-type moves like Psybeam or Confusion are also super effective against Gengar. However, Gengar can use its Ghost typing to avoid these moves as well.

3. Dark Type: Dark-type moves like Crunch or Bite can also deal significant damage to Gengar. It is important to note that Gengar's Poison typing does not protect it from Dark type moves.

Overall, Gengar's weaknesses can be exploited by using Pokémon with these types of moves during battles.