Bug and fighting types.
Ghost-Types are weak against Dark-Types and other Ghost-Types.
The Pokemon can only be hit by "super-effective" moves. For example, Shedinja can only be hit by fire, rock, ghost, dark, and flying type moves. If a Pokemon tries to attack it with a water type move, Shedinja can't be hit because of it's wonderguard.
Psychic and Ground type moves are super effective against Poison type Pokémon.
There are many different dark Pokemon in Diamond. To find a complete list of them you can go here: http://www.serebii.net/pokedex-dp/dark.shtml It lists all of the dark Pokemon from all of the games.
=====================DARKRAI===========
Dark type moves such as night shade,shadow punch,etc.are super effective against dark Pokemon.
Ghost and Dark attacks are super effective against Ghost type pokemon.
Ghost and Dark-type Pokemon are super effective against Ghost types.
Both Fighting types and Bug types are super effective against Dark types.
Darkrai = Dark-type, Fighting is super effective against dark.
Fighting and bug type moves are super effective against the dark type.
Fighting and Bug
Fighting and Bug
If your Reffering to pokemon,its fighting a bug
Fighting and Bug types are super effective.
dark, for one, but psychic is super effective against dark to, try doing some research, it will be easy to find out.
Ghost and dark.