you need to train them, give them massages or give them poffins and irons ect.
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there is no best way there is only your way.
The best way is challenge the elite four with your Pokemon or a Pokemon holding exp. share. The better is to use gameshark codes for rare candies.
The best psychic Pokemon on Pokemon diamond is alakazam. It can beat an infernape level 100. who carez about a dull looking Pokemon with two unusually big mustaches... ew... but I like Lugia and Espeon... and Gallade... and Gardevoir... bronzong all the way haha
you use diffrent types of Pokemon to get a advantge
It depends if you have a way you can cheat or not