I know that sounds harsh, but Dusknoir is kind of more built for the Meta-game than in-game. There is a lack of good Ghost types in Hoenn, but I guess you can always trade for one. If not, I think that (Mega) Bannette [**At 555 BST over Dusknoir's 525**] is a better Pokemon overall.
Also, getting Dusknoir is a kind of a difficult process (Unless you already have one, that is,), what with trading a Dusclops that's holding a Reaper Cloth (meaning you'll have to find one too).
Also, it is better to have a Dark type because you'll be at a disadvantage for two of the four elite $ matches (Dark and Ghost).