The 5th Gym Badge is in Pastoria. You have to beat Crasher Wake which you met when you were going to the 4th gym. He specialises in water Pokemon. He has Gyrados, Quagsire and Floatzel. They are all in the mid 30's. The best type of Pokemon to use against Gyrados is an electric Pokemon. Against Quagsire you should use a grass Pokemon. On Floatzel you could use either grass or electric Pokemon.
The fifth gym leader in Pokemon platinum is crasher wake but you get the fourth badge.
The fifth gym leader is Fantina and she uses Ghost Pokemon. No element is strong against ghost except ghost, so just use your strongest Pokemon. But be careful - ghost to ghost is super powerful, but unless your ghost Pokemon is a much higher level than hers, do not use it since her attacks will be super effective against you. The best Pokemon to have are just a well-rounded team with most of the elements covered.
Give the hm or TM to your Pokemon
First of all, badges get you into the Pokemon league. The coal badge in paticular, lets you use HM rock smash outside of battle.
The 8th Badge in Pokemon Diamond is called The Beacon Badge.
In Pokemon Red, the fifth badge is possessed by the Poison-Type Koga. The badge is named Soul Badge. It lets you use Surf outside of Battle.
you should use either ground or psychic Pokemon
Hearthome
hearthome city
Fuschia city.
Hearthome City.
Petalburg City.
Beat the gym leader
The fifth gym leader in Pokemon platinum is crasher wake but you get the fourth badge.
You need to defeat Koga in Fuchsia City to get the 5th badge, the Soul Badge.
The fifth badge is in Sootopolis City, you need Surf, Dive, and Whirlpool to get it
Trade a Pokemon from an old version and make sure you have the fifth badge.