How do you get past the Psyduck on route 210 in Pokemon?

you go to pastoria city look by the great marsh entrance and you should see a galactic grunt. chase him. he will resist battle until he is too tired to go on. fight him and beat him. then Cynthia should come up to you and give you the secret potion. it is a key item. go up to the psyduck and press A. it will ask if you want to use the secret potion. say yes. they will move and you can go to celestic town.