You have any psychic pokemon? Kadabra, Drowzee, Alakazam, etc? They are good for fighting ghost types. Elite Four Agatha uses ghost-type pokemon and the only way to defeat them is to use psychic. Well if you already gotten through the four, I recomended you Onix. The sevii islands contain many rocks. So, you have to get at least Strength and Rock Smash and teach 'em to Onix.
Level him up until at least lvl 70, like my Onix (It's now 82, though).