Easy way to defeat all gym leaders - Guide for Pokemon Diamond

Roark: With Piplup as your starter, you can beat Roark with ease if it is above 
level 12. If you already have Prinplup, it will be ultra easy.
Gardenia: Catch a Buizel at Valley Windworks, and then you can trade it in 
Gardenia’s town (I forget the name) and have a Prinplup know Peck. Be careful 
of her Roserade, though.
Maylene: If you have a level 30 Flying type and a fighting type, it will be 
very easy.
Crasher Wake: Have an Empoleon for the resistance, and have a Raichu for the 
offense and it will be easy.
Fantina: This one is a little hard, but if you have Empoleon that knows a 
strong water-type move (Brine is recommended), it will make it a little easier, 
and if you have a Drifblim that knows Ominous Wind, it will be a little easier.
Byron: If you have Empoleon, Machoke, and a good Fire type, you’re unstoppable.
Candice: This one is very hard if you have Torterra, but if you have Infernape, 
you’re good for the majority of the battle. Just switch to a flying type when 
she brings out her Medicham.
Volkner: This battle is very hard, but if you keep your ground with a good 
Ground type or a Dialga that knows Earthquake, you’re good for the battle. If 
you have a Ground type, switch to an electric type when he uses Octillary.