INCORRECT. :[
Good fire types are so hard to come by nowadays. Pearl had Houndoom, but Diamond is devoid. >___>
I was gonna use Rotom, but you can only get it after the National Pokedex in Diamond, along with a bunch of other things I was thinking of.
Muhh.. I have most weaknesses in Bug, Poison, Flying, and Dragon. A Steel type seems to cover those. Here's the list of Pokemon that are Steel:
Empoleon, Steelix, Bastiodon, Bronzong, Lucario, Probopass, Magnezone, Scizor.
Can't do Empoleon, Bastiodon is Pearl only, I already have a Psychic type for Bronzong, Nosepass and Magneton are only after the National Pokedex, and Scyther is very late in the game.
That leaves Steelix and Lucario.