I threw this together before actually getting Sword, but I honestly had this same thought! Put this together, intending for Doubles.
Drapion @ Black Sludge
(ability irrelevant, Sniper I suppose)
-> Toxic Spikes (obviously)
-> Whirlwind (obviousy)
-> Knock-off (remove Lefties, etc)
-> Cross Poison (anti-Taunt, chance to Poison Flying and Levitate users)
Careful, 248 HP, 38 Def, 222 SpDef (equal defenses at 265)
Crustle @ (Lefties I guess?)
Sturdy
-> Spikes (obvious)
-> Stealth Rock (obvious)
-> X Scissor (filler, powerful STAB)
-> Stone Edge (filler, powerful STAB, hits fliers super effective)
Careful, 252 HP, 252 SpDef, 4 free (SpDef can't catch Def, so max health)
Milotic @ Lefties (Flame Orb probably better?)
Cute Charm? (Marvel Scale probably better?)
-> Dragon Tail (switch + damage, yes please)
-> Coil (setup for Def and Atk)
-> Waterfall (filler STAB, confusion chance helpful)
-> Life's Dew (heal self and ally)
Impish, 252 HP, 252 Def, 4 free (Def again can't catch SpDef, but much closer. Math gets weird with Coil, more so if Flame/Scale)
Kommo-o @ Lefties
(ability irrelevant, Overcoat if you're expecting Hail/Sand)
-> Dragon Tail (switch + damage)
-> Drain Punch (STAB, sustain)
-> Substitute? (more sustain)
-> Stealth Rock? (back-up application)
Careful, 252 HP, 104 Def, 152 SpDef (equalizes defenses at 312, maxes health for longevity)
Ferrothorn @ Lefties
Iron Barbs
-> Protect
-> Ingrain
-> Iron Head
-> Seed Bomb
Careful, 168 HP, 172 Def, 168 SpDef (equal defenses at 341)
I was grasping at straws for a 6th, so I threw together a Butterfree (still has Whirlwind) to work as a stall medic (Pollen Puff, Protect).
Simply put, use Drapion and Crustle first for setup. Sadly, Crustle doesn't know any switching moves, but it is heavily defensive with both Spikes and Stealth Rock. STAB Stone Edge is also rather helpful for dealing with Flying types. Drapion will probably not survive that long, but will at least get out Toxic Spikes. Something pretty important to note, is that you only want to put 1 layer out, not 2. Normal Poison does more damage the first turn than Toxic Poison does, and the intention here is to force the opponent to switch via Dragon Tail, Roar, and Whirlwind as much as possible to force more Stealth Rock and Spikes damage. Personally I'd hold Ferrothorn as long as possible, and use it in a bit of a clean-up capacity. After you've completely disrupted the enemy's whole team with all the force switching and applied Poison to everything, now you're throwing down one of the most irritating tanks in the entire Pokedex for them to deal with, abusing both Lefties and Ingrain alongside Iron Barbs for retaliation. Normally you'd do Leech Seed, but if you still have one of your switchers up it's not going to do you any good, since it gets removed when the victim switches out.
Kommo-o gives you an offensive option for when the passive tricks don't work. You've got room in the movepool to set up a lot of different things. Want an additional Stealth Rock in case the enemy has a Rapid Spinner? Easy. Want to bring Protect to stall a little? No problem. Want Dragon Dance or Work Up for a physical setup? Done. Options abound for you to do some different things. Milotic ends up being similar. Milotic also ends up being a bit of a surprise, since most sets featuring it use SpAtk. But since we're bringing Dragon Tail, we're going physical instead. This works great with Coil, since it raises your Atk along with your Def and accuracy. If you find yourself against a Fairy, you've got Waterfall for STAB and the chance for confusion. And Life's Dew helps not just Milotic itself, but also your other Pokemon (remember, this team had Doubles in mind).
