I'm posting my team for future reference.
I went 16-0 on Pokemon X, rating 1702. However, many of my opponents had a lower rating, so I'm not sure if this team is really that good.
Kangaskhan@Kangaskhanite
Ability: Scrappy
EV: 252 Atk,/252 Speed
Jolly
-Power-up punch
-Fake out
-Return
-Sucker punch
Garchomp@Lum Berry
Ability: Rough Skin
EV: 252Atk/252 Speed
Jolly
-Outrage
-Poison Jab
-Earthquake
-Swords Dance
Poison Jab for opposing Garchomps. Originally it was holding Life Orb, but I changed it to Lum berry because paralysis and burn were very common. It also cures confusion from outrage.
Serperior@Focus Sash
Ability:Contrary
EV: 252 SpAtk/252 Speed
Timid
-Leaf Storm
-Dragon Pulse
-Glare
-Gastro Acid
Gastrodon@Sitrus Berry
Ability: Storm Drain
EV: 252 HP/ 252 Def
Relaxed
-Scald
-Recover
-Earth Power
-Ice Beam
My answer to Serperior. Also, it can deal very well with other water types, because it's not weak to water/ice.
Porygon2@Evolite
Ability: Trace
EV: 252 HP/ 252 Def
Bold
-Tri Attack
-Thunderbolt
-Recover
-Foul Play
Thunderbolt for Garchomp. Foul Play was fun, I managed to KO a DD Tyranitar thanks to it. In retrospect I think that I could have changed Tri Attack or Foul Play to Toxic.
Avalugg@Rocky Helmet
Ability: Sturdy
EV: 252 HP/ 252 Def
Relaxed
-Avalanche
-Recover
-Toxic
-Mirror Coat
My answer to Kangaskhan.
A pretty standard team. Kangaskhan, Serperior and Garchomp were sweepers, capable of dealing with 2-3 pokemon when not stopped. Avalugg, Porygon2 and Gastrodon were meant to counter specific threats.
I really enjoyed this format, and I hope it's going to be used again in the future, this time as a double battle.