Meditite's weaknesses:
* Fighting: Meditite is a Psychic-type Pokémon, and Fighting is super-effective against Psychic.
* Ghost: Ghost-type moves are also super-effective against Psychic-type Pokémon.
Moves to use:
* Fighting-type: Brick Break, Low Kick, Aura Sphere, Close Combat, Cross Chop
* Ghost-type: Shadow Claw, Shadow Ball, Lick, Shadow Sneak
Important Note: Meditite can evolve into Medicham, which is both Psychic and Fighting type. This means that it will be resistant to Ghost-type moves.
Let me know if you have any other questions about Pokemon Pearl!