Unusual Mixed Set
Stats: Adamant/Jolly - 252 Speed & Attack, 4 Sp.Atk
Ability: Perversity
Moves: Leaf Storm, Iron Tail, Aerial Ace, Hidden Power (Ice/Ground)
Item: Choice Scarf, Metronome, Expert Belt, or Leftovers
Aerial Ace's average power will be aided by Iron Tail's chance of an added Defense cut (barring Clear Body, White Smoke, or Big Pecks Abilities) and by Intimidate Ability opponents. Leaf Storm will be useful while it lasts (Pressure Ability screws with the low PP) while HP (Ice) will rip into many Dragon Types while HP (Ground) handles Sap Sipper Zebstrika.
Serperior may scream "Perversity plus Leaf Storm," but Physical-Weak opponents that resist grass will be in for a surprise from Mixerperior (especially those with Intimidate).
Problem Opponents
--Anything with Sap Sipper should be a priority problem to tackle.
--Special Serperior should fear Pressure Ability opponents. Leaf Storm's 5 PP will be gone faster than you can say "Perversity."
--Physical Serperior should fear Intimidate, faster opponents with Choice Band/Specs (Accelgor, Aerodactyl, Weavile, etc.), and par-Speed opponents with Choice Scarf (Scolipede, Gengar, Special Technician Persian, etc.). Modest Technician Persian with Icy Wind (4th Gen Tutors) will be an issue without Perversity in play.
--Mixerperior should fear Tanks, Stallers, and Pokemon with Sap Sipper, Clear Body, White Smoke, or Big Pecks Abilities.
Potential Counters
--Sap Sipper Zebstrika with Flame Charge can do a ton of damage to Serperior. This one is a huge problem because Serperior can only hit it with neutral hits, barring Hidden Power (Ground).
--Heat Rotom can be problematic with resistances to many of Serperior's Moves.
--Charizard and Scarf-Typhlosion can ruin Serp's day.
--Slip Through Crobat can give Stallerperior hell.
--Blissey, Volcarona, and Snorlax can be problematic for Special Sets.
--Ferrothorn's Gyro Ball can be a threat to the Speed-heavy Serperior and it can be a threat in general to Physical Sets.
--Whimsicott can prioritize-stall over Serperior.
--Escavalier and Spiritomb can stall offensive sets for a respectable duration, plus Vespiquen can do so for longer with Roost/Heal Order and Toxic.
--Accelgor (Bug Buzz), Jolteon (Signal Beam from 4th Gen), and Aerodactyl (STAB Flying) can unleash serious hurt on a non-Scarfed Serperior.