What does your SoulSilver team consist of? If we could gather info on it, it'd likely be more easy to lend suggestions.
Anyhow, one thing I would recommend trying, first and foremost, is to perhaps remove Quick Claw from Dusclops, and give Eviolite. Dusknoir's very tank-y, and its pre-evolution could be even more do, in a way, if it was given this particular item to hold. I've tried Eviolite Dusclops a couple of times before; it was decent, I would say!
As for your third team member, Mega-Gardevoir does sound like a good option. Alongside, another Pokémon we could consider carrying is Togekiss. With para-flinch strategy, it has chance to wreak some major havoc.