Defensive (Koffing) @ Leftovers  
Ability: Levitate  
EVs: 248 HP / 252 Def / 4 SpD  
Impish / Relaxed Nature  
- Pain Split  
- Sludge Bomb  
- Hidden Power [Grass] / Taunt
- Will-O-Wisp
Optional Move(s): 
- Explosion
- Haze
- Memento
- Fire Blast
Optional Item(s): 
- N/A
Optional Ability: 
- N/A
Optional EV/IV(s): 
- N/A
Overview: Koffing shines in ADV ZU thanks to its great physical bulk, Levitate, and defensive typing, which grants it immunities to both Ground-type moves and Toxic, and is one of the best counters to Rhyhorn available. 
Defensive: Koffing is able to fill a role that other physical walls cannot, checking Rhyhorn and threatening Pidgeotto and Taillow. This makes Koffing an extremely valuable Pokemon defensively. Pain Split works in conjunction with Leftovers and its low base HP to keep Koffing healthy. Hidden Power [Grass] allows Koffing to more effectively answer Rhyhorn. Taunt can be used over Hidden Power [Grass] to shut down opposing setup opportunities as well as shut down other defensive Pokemon. Will-O-Wisp allows Koffing to threaten key physical breakers such as Aipom, Pidgeotto, Taillow, and Rhyhorn.
Other Options: Koffing can run Explosion over Pain Split to OHKO a frailer Pokemon that cannot OHKO Koffing such as Elekid, Pidgeotto, and Aipom. Haze can be used to prevent set-up sweepers such as Delcatty, Grimer, and Larvitar from achieving their sweep. Memento is appreciated by set-up sweepers such as Growlite, Rhyhorn, and Croconaw. Fire Blast allows Koffing to threaten certain Pokemon that may switch in and otherwise wall Koffing such as Magnemite and Dustox.
Checks and Counters: Special attackers such as Elekid, Staryu, and Seaking can all threaten Koffing with their STAB attacks. Staryu is worth noting because it can absorb Koffing's Will-O-Wisp and Recover any lost damage. Rhyhorn is able to use Substitute in front of Koffing that lack Hidden Power [Grass] and block Will-O-Wisp and Pain Split. Similarly, Larvitar is able to absorb Will-O-Wisp with Guts and proceed to set-up if Koffing lacks Hidden Power [Grass]. Magnemite is immune to Koffing's Sludge Bomb and can heavily damage it back with Thunderbolt.