Weezing, the premier defensive poison type... in Gen 3 where there were no super-effective physical moves to strike it. After that, Weezing has been on a downward spiral, as the game diversified and everything got stronger. Not that it still doesn't do its job well in the lower tiers.
An Old Gas Bag
Weezing
Ability: Levitate
Item: Leftovers
-Will-o-Wisp
-Sludge Wave
-Pain Split
-Haze/Taunt
Nature: Bold (+DEF, -ATK)
EVs: 252 HP/252 DEF/4 SDEF
Tried and True Weezing. Burn any physical sweepers, recover with Pain Split, null any setup with Haze or stop setup entirely with Taunt, while STAB Sludge Wave prevents Weezing from being entirely passive.
Some damage calcs:
252+ Atk Choice Band burned Copperajah Heavy Slam (120 BP) vs. 252 HP / 252+ Def Weezing: 102-121 (30.5 - 36.2%) -- guaranteed 4HKO after Black Sludge recovery
252 Atk Life Orb Reckless Hitmonlee High Jump Kick vs. 252 HP / 252+ Def Weezing: 99-117 (29.6 - 35%) -- guaranteed 4HKO after Black Sludge recovery
252 Atk Gallade Zen Headbutt vs. 252 HP / 252+ Def Weezing: 164-194 (49.1 - 58%) -- 58.6% chance to 2HKO after Black Sludge recovery
252 Atk Life Orb Sheer Force Kingler Liquidation vs. 252 HP / 252+ Def Weezing: 149-177 (44.6 - 52.9%) -- 0.4% chance to 2HKO after Black Sludge recovery
252 Atk Passimian Knock Off (97.5 BP) vs. 252 HP / 252+ Def Weezing: 64-76 (19.1 - 22.7%) -- possible 5HKO
In short, Weezing can switch into most of RU's premier physical threats, take another hit and then Burn them into uselessness (Weezing naturally outspeed Copperajah who otherwise 2HKOs). Note that Weezing does not have the special bulk to even attempt pulling off the same against them.
Other Options
Toxic Spikes -Entry Hazards are nice, but this can prevent Weezing from burning select targets.
Explosion/Destiny Bond - can take out an unsuspecting target, but again one must question the wisdom of blowing up your own wall.
Clear Smog -Taunt Proof haze that doesn't hit Steel types
Countering Weezing
Fire types are a huge problem for Weezing, as they can't be burned. While Weezing can threaten most thanks to STAB Sludge Wave/ Toxic, Coalossal resists poison and can beat Weezing down in turn, but note that Coalossal can't use Weezing as setup fodder, as Taunt, Haze and Clear Smog with stymie most setup attempts. Guts users like Gurdurr don't mind Burns and can eat a suspected Will-o-Wisp to their advantage (though Gurdurr has difficulty breaking through Weezing even with Guts active). It is best to treat Weezing like Skarmory of old, surprise it with a Special move to remove it from the match, cause it won't come out unless it knows it is safe.