Like sumwun said, there is really no way of positively knowing which Format or Tier you want this for.
I am rating this team on OU. If OU isn't the tier you were going for, disregard this advice.
If you want to keep all of the members of your team, there are a few suggestions that I would make.
Calm Mind. It isn't needed with the set you currently have. I also suggest getting rid of rest. Switch Night Shade out for Seismic Toss. (which Dusclops can learn) Keep Will-O-Wisp. It completely cripples any and all physical attackers. You'll also want to devote his EV's toward Defense to more easily take on those Pokemon.
I also suggest giving him Shadow Sneak and Taunt. (Shadow Sneak might get an unexpected KO (I killed a Mega-Sharpedo with it), and Taunt let's him shut down the inevitable set-up Mon.
With Evolite, he's even bulkier than Dusknoir, however, he heavily relies on the Evolite to keep himself alive. I suggest Switching Dusclops out for Dusknoir. Yes, he's slightly less bulky, but he can carry leftovers, and increase his life span. Also, his higher attack will make Shadow Sneak a little stronger.
That Smogon set, tho.
The only change I'd make would be to switch Toxic Spikes for Toxic. You already have a pretty reliable hazard setter in Forretress. True, Toxapex is MORE reliable, but you don't really need it to carry more spikes. Also, it will help you be less passive, and guarantee status, instead of Scald's 30% chance to Burn.
He's fine. Good coverage. I would suggest switching it's EV's to a more speed-centered spread.
Remember to only bring him in on weakened opponents, tho. He isn't strong enough to OHKO every threat. Spikes support will also help him a lot.
I don't really have any suggestions for this one. I was unable to use it effectively. (blame my terrible battling skills)
He's fine. Does his job pretty well.
Honestly, the only problem I have with this guy is the Air Balloon. Sure, he can switch into a predicted Earthquake, but once he's there, he can't do anything. He can try to set up, but most Pokemon with Earth Quake have another attacking move, to pop the Balloon. Also, Drapion's attacks are not very effective on common earthquake users. He's walled.
Get rid of his air balloon, give him a Sitrus Berry, a Lum Berry, or Leftovers.
Get rid of Swords Dance. Replace it with roar for Phazing.
Here it is:
Dusknoir @ Leftovers
EVs: 252 HP / 252 Def
- Seismic Toss
- Shadow Sneak
Toxapex @ Black Sludge
EVs: 252 HP / 252 SpD
IVs: 0 Atk
Aerodactyl-Mega @ Aerodactylite
Ability: Tough Claws
EVs: 252 Atk / 252 Spe
- Iron Head
- Ice Fang
- Rock Slide
Lilligant (F) @ Life Orb
EVs: 252 SpA / 252 Spe
IVs: 0 Atk
- Quiver Dance
- Sleep Powder
- Energy Ball
Forretress @ Rocky Helmet
EVs: 252 HP / 4 Def / 252 SpD
IVs: 0 Spe
- Stealth Rock
- Toxic Spikes
- Rapid Spin
- Gyro Ball
Drapion @ Lum Berry
Ability: Battle Armor
EVs: 188 HP / 120 Atk / 200 SpD
- Knock Off
- Poison Jab
Those are my suggestions. Hope it helps!