To beat Gengar, you should take advantage of its type weaknesses, which are Dark, Ground, Ghost, and Psychic. Here are some specific strategies and counters:

• Alakazam: This Psychic-type Pokémon is a good counter for Gengar.
• Mewtwo: Another strong Psychic-type Pokémon that can be used to beat Gengar.
• Tyranitar: This Dark-type Pokémon is a good counter for Gengar.
• Metagross: This Psychic/Steel-type Pokémon can be used to beat Gengar.
• Groudon: This Ground-type Pokémon is a good counter for Gengar.
• Chandelure: This Ghost/Fire-type Pokémon can be used to beat Gengar.
• Rhyperior: This Ground/Rock-type Pokémon is a good counter for Gengar.
• Espeon: This Psychic-type Pokémon can be used to beat Gengar.
• Shadow Mewtwo: This Psychic-type Pokémon is one of the best counters for Gengar.

Its also worth noting that Gengar is weak against a mixture of strong ground and psychic-type Pokémon.