SubStatus (Magnemite) @ Leftovers  
Ability: Magnet Pull  
EVs: 4 HP / 252 SpA / 252 Spe  
Timid Nature  
IVs: 2 Atk / 30 SpA / 30 Spe  
- Substitute  
- Thunderbolt  
- Hidden Power [Grass] / Hidden Power [Fire] 
- Toxic / Thunder Wave
Optional Move(s): 
- N/A
Optional Item(s): 
- N/A
Optional Ability: 
- N/A
Optional EV/IV(s): 
- 252 HP / 4 SpA / 252 Spe
Overview: Magnemite boasts a base 95 special attack stat, access to STAB thunderbolt, and is one of the only usable Steel-types in ADV ZU. This allows Magnemite to shine as not only a special attacker but as valuable utility. 
SubStatus: Substitute provides Magnemite an option for scouting switches. It also finds many opportunities to use Substitute thanks to Pokemon like Delibird and slower passive Pokemon that rely on Toxic like Lileep. Hidden Power [Grass] allows Magnemite to reliably beat Rhyhorn and chip other Pokemon that are immune to or resist Thunderbolt such as Chinchou and Cubone. Hidden Power [Fire] can be used instead to check Grass-types like Exeggcute and Bayleef. Toxic works well with Substitute to rack up poison damage on Pokémon like Lileep. Thunder Wave is an option to hinder offensive switch-ins and wallbreakers.
Other Options: 252 HP EVs can be used to help check Pidgeotto and Tailow somewhat more reliably as Double-Edge from Pidgeotto only has a 19% chance to 2HKO. 
Checks and Counters: Nosepass can trap and remove Magnemite with Earthquake thanks to Magnet Pull. Cubone can threaten to OHKO Magnemite through Substitute with Bonemerang. Pokemon that run Fighting-type coverage such as Pidgeotto, Aipom, and Taillow can all 2HKO Magnemite. Aipom is worth noting due to it having a 19% chance to OHKO with Brick Break. Rhyhorn is able to force Magnemite out if it lacks Hidden Power [Grass] which leaves room for Rhydon to set up a Substitute. Special walls such as Sunflora, Chincou, and Lileep are able to shut down Magnemite, though none of them appreciate Toxic. Finally, specially-based Castform is able to threaten Magnemite out with Flamethrower.