A fast fighting type like infernape. because wevile is dark/ice type making it four times as effective. But look out, Weaville is fast.
Darkrai = Dark-type, Fighting is super effective against dark.
Flying and psychic
i would say Garchomp, Empoleon, Wevile, Rapidash, Ryperior and Honchkrow you could switch empoleon for gyarados and Rapidash for infernape if you want
To get weaville on DP you need to catch a sneasel. (their pretty common) Then give it your Razor Claw to hold and give it a rare candy or level it up and it will evolve.
The dark type is super effective against it, plus psychic can't hurt them, and the bug type is super effective, but it takes psychic attacks as a normal effective hit. I would suggest using beautifly or kricketune, since there isn't any murkrow in Pearl.
Fantina uses Ghost types. Ghost type moves are not very effective against Dark and Steel types. Dark and Ghost type moves are super-effective against Ghost types.
Gastly is a ghost/ poison type, so any psychic or ghost type will be super effective against it.
Fire
He has a toterra,electabuzz,honchcrow,and a usaring, gliscor, wevile, magmar. also, he has a nidoking and a hariyama. (seen in Pokemon dp ep 128) so probably till the endof the sinnoh league , he'll have a torterra, electabuzz (if it don't evolve), magmar (if it don't evolve), honchcrow, ursaring, gliscor, wevile. nidoking and a hariyama.
Psychic is super effective and normal has no effect against eachother with ghost
The expert belt in Pokemon diamond and pearl increases the power of a super effective attack.
flying type and ghost type Pokemon