in petalburg city
Go beat the 6th gym leader ^^, that is in Fortree City.
after you beat 4 to 5 gym leaders you can face him
Simply go inside.
its the petalburg gym and/or gym 5.
petalburg gym
in the gym
the fifth gym on Pokemon diamond is the one in herthome city,
The 8th gym can be found in Sootopolis. Who is managed by Juan(uses water type). Hints: Juan has 5 pokemons which is: Luvdisc(be careful by its ATTRACT) Kingdra Sealeo Whiscash crawdaunt
Gym 5 is located in Safaron City, and the Gym Leader is Sabrina. She uses Psychic type Pokemon, and it's a bit of a puzzel to get to her.
I don't know use Magikarp
In order to get the HM Surf, you must obtain 5 badges. Wally's father gives it to you after you defeat Norman (Petalburg City gym).You will get HM surf after you win petalburgh city gym badge.its in your vag
beat him with your pokemon!