SpDef Wall (Dustox) @ Leftovers  
Ability: Shield Dust  
EVs: 252 HP / 240 SpD / 16 Spe
Careful Nature
- Toxic   
- Sludge Bomb
- Whirlwind / Protect
- Moonlight  
Optional Move(s):
- Thief
- Psychic
- Hidden Power [Grass]
Optional Item(s):
- N/A
Optional Ability:
- N/A
Optional EV/IV(s):
- N/A
Overview: Dustox is unique as a special wall thanks to its Toxic immunity and its access to Whirlwind, the former of which lets it set itself apart from other options like Lileep, Bayleef, and Sunflora. 
SpDef Wall: Toxic allows Dustox to punish opposing switch-ins and the Special Attackers it walls. Lileep is an easy target for Toxic due to its tendency to switch in to wall Dustox and a super effective Rock Slide. Sludge Bomb means that you are not helpless against the Grass-types you are meant to counter. Alternatively, Hidden Power [Grass] allows Dustox to avoid becoming setup fodder from Substitute Rhyhorn. Whirlwind prevents opposing Pokémon such as Rhyhorn and Growlithe from setting up on Dustox, but Protect is an option to scout versus choiced-locked wallbreakers, especially Aipom, Pidgeotto, and Taillow, as well as to gain more Leftovers and to rack up Toxic damage. Moonlight provides Dustox with fairly reliable recovery, which bolsters its ability to perform as a wall, despite its low PP. The allotted Speed EVs means you will always outspeed Adamant max Speed Cubone. 
Other Options: Dustox can choose to forgo its item and use Thief over Whirlwind in order to steal Leftovers from other Dustox, Koffing, Magnemite, and other bulky Pokémon that may try to switch in. Psychic and Hidden Power [Grass] are both options over Sludge Bomb or Whirlwind to hit other Poison-types like Koffing and opposing Dustox, and Rock- and Ground-types like Rhyhorn and Cubone, respectively. When choosing these over Sludge Bomb, make sure to run a Calm Nature instead of Careful. 
Checks and Counters: Lileep is able to wall Dustox regardless of which attacking move is chosen and force it to waste Moonlight PP with Rock Slide, though it does not appreciate Toxic. Koffing and Magnemite are able to completely counter Dustox and status it. Physical wallbreakers such as Aipom, Pidgeotto, and Taillow can all OHKO Dustox. Taillow in particular is worth noting, as Toxic boosts its attack via Guts. Both Cyndaquil and Growlithe are able to threaten Dustox with their Fire-type STAB.