Here's my opinion:
For starters, Dusknoirs HP is horrendous. Machamp could run through it without too much difficulty, despite the immunity. It would just take a bit more tapping. Mismagius works, and it even has Dazzling Gleam which will certainly surprise Machamps, Rayquazas and Tyranitars alike. Unfortunately, Metagross resists this move, which makes it a viable counter. (which is what we didn't want) Dark Pulse and Shadow Ball work great, but I would think Shadow Ball would come out on top as the reluctant people who didn't want to switch from MC would resist Dark Pulse. Drifblim is really niche and only works against certain Pokemon, so a generalist team could blow by it fairly easily. But it would really anger people who brought in only Machamp. It does have coverage against Dragons from Icy Wind, which is a cool bonus. (Unfortunately, Metagross resists this move, so yadda yadda yadda) There is one flaw in this design however. Say you had Blissey-Gardevoir-Chansey-Drifblim-Snorlax-Mismagius. Just have 3 Dark Ttars and 3 Machamp.
The Slowdudes wouldn't make any difference, as Ttars hit them for Super Effective damage anyway, as well as the fact that Water Pulse is the definiton of sh*t.