It depends on the psychic, There is a few tricks you can do, if for instance your playing a guessing game with the psychic person, you pick a card from a deck of cards, and you look at it quickly, then your supposed to sit there and think of the card you just chose but not tell the psychic who is guessing, instead when you see your card immediately start thinking of a completely different card as many times in your head and as fast as you can to block the real information from coming through to them, this works about 80% of the time, unless the particular psychic is fast enough to pick up on that quick change of thought... if your trying to "defeat" the psychic by means of hurting them mentally or physically (for instance your with someone who is psychic and you cheated on them), then good luck to you, most of the time that is information that is always on the fore front of the psychic's mind, to protect them selves from harm, it is extremely difficult to lie to or trick a psychic, you may think you have won, but they know the truth, they probably aren't letting you know that they know the truth.
Psychic or Ground type attacks.
Psychic
use a very strong psychic type that has very powerful psychic attack
That's easy, use psychic or ground pokemon. But psychic are the best.
use dark and psychic. normal moves do 0 damage
Use psychic, electric, or rock moves.
a flying or psychic move should do the trick.
Use a high level flying type or psychic type.
psychic, dark, flying, ghost
He's fighting type? Use psychic or flying 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.
Try to use a flying Pokemon or psychic Pokemon, I used Ralts and Taillow to beat him. Yeah so try using flying or psychic.