Use a lucky egg :D if you dont have one look for a chansey in wild and use thief on it I think there's a 50% chance you get one, and if you dont know what the lucky egg does it doubles the amount of xp you recieve in after a battle if you dont wanna try that you can always make a team made up of legendaries put a exp share on relicanth and beat the E4 over and over again. well thats how I train my Pokemon in platinum.
Good luck with relicanth!
