potentially, yes.
For instance, Gyarados can learn Dark Pulse (Dark moves are super effective on psychic Pokemon) and Gyarados is a Water type.
Also, neither Pokemon will really be super effective on the other (if they only have water and psychic moves, respectively, and are not of two types), so the stronger, better trained Pokemon will probably win, or they will each have an equal chance.
Yes it can, but it will appear not as effective as hitting the psychic type with a bug, dark, or ghost type. Try teaching your water type pokemon one of those move types.
A water and psychic type pokemon such as Slowbro will be weak against the following types: Electric, Grass, Dark, Bug, and Ghost.
Pyschic type beats pyschic type.
dark,ghost,etc
A Poison/Ground-type Pokemon is weak to Psychic, Ground, Water, and Ice-type attacks. Nidoking and Nidoqueen are the only two Pokemon with this typing.
I would choose a Dark or Ghost type
Ground types and psychic types beat poison types.
Gothita is a Psychic type pokemon.
Lorelei has Ice and Water type pokemon, the best way to defeat her pokemon is to use Electric type pokemon or pokemon that have Electric type moves. Bruno has Fighting type pokemon and two Rock/Ground types, the best way to defeat his pokemon is to use Psychic pokemon or pokemon that have Psychic type moves. Agatha has Ghost and Poison type pokemon, the best way to defeat her pokemon is to also use Psychic type pokemon or pokemon with Psychic type attacks.
Pokémon types that can beat Psychic-type Pokémon are Dark, and Bug. Ghosts however lose against Psychic.
Ghost and Psychic
Fighting-type pokemon are weak against it.
Psychic
Psychic and Ground
use psychic Pokemon and beat all the trainers to get more light
You can beat it easily with Dark, Bug, and Ghost type Pokemon. It is least effective by Psychic, and Fighting.
dark types or ghost types
Ground
use psychic Pokemon and beat all the trainers to get more light
A Poison/Ground-type Pokemon is weak to Psychic, Ground, Water, and Ice-type attacks. Nidoking and Nidoqueen are the only two Pokemon with this typing.
Try using flying and psychic types ;)