Meowstic @ Rocky Helmet
Ability: Prankster
EVs: 252 HP / 204 Def / 4 SpA / 44 SpD / 4 Spe
Bold Nature
- Gravity 
- Safeguard
- Quick Guard
- Psychic
Meowstic uses gravity to allow earthquake to hit Levitate/Flying Pokémon and to increase the accuracy of Rock Slide, also uses Safeguard to protect from status and Quick Guard for Fake out and other priorities.  Use gravity turn one unless there’s an expected Fake out, in which case use quick guard.  Rocky Helmet was because I had no idea what else to use, it has proven too useful to bother with a replacement.

Terrakion @ Kee Berry 
Ability: Justified
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Rock Slide
- Earthquake 
- Protect
- Sacred Sword
Terrakion goes in the middle and just spams Rock Slide and occasionally Earthquake, although for fast threats like Alakazam on turn one, use Protect and have Whimsicott and Meowstic use Tailwind and Safeguard.  Terrakion's Kee Berry is for a +1 to Defence.

Whimsicott @ Kebia Berry
Ability: Prankster 
IVs: 0 Atk 
EVs: 252 HP / 4 Def / 52 SpD/ 200 Spe
Timid Nature
- Beat up
- Energy ball
- Moonblast
- Tailwind
Whimsicott beats up Terrakion to activate Kee berry and get a +6 in Attack from Justified, also uses Tailwind for team speed.  Beat up Terrakion turn one unless there’s a faster threat like Alakazam, then use Tailwind instead and beat up Terrakion on turn 2.  Whimsicott's Kebia berry is for Crobat.  

Weavile @ Focus Sash
Ability: Pressure
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Fake out
- Knock Off
- Ice Punch
- Protect 
Once any of them faints, Weavile comes in for Fake Out, distraction or just damage with Ice Punch and Knock Off. After losing something else, you bring out Gardevoir or Salamence based on whichever is more likely to be useful in the current situation. I usually go for Gardevoir  

Gardevoir @ Gardevoirite
Ability: Pixilate
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Hyper Voice
- Psyshock
- Hidden Power Ground
- Protect
Hyper Voice with Pixilate is pretty strong, Psyshock hits Pokémon with lower physical defence nicely, Hidden Power Ground is mostly just extra coverage for things like Heatran and Protect is always useful in doubles/triples formats. 

Salamence @ Salamencite
Ability: Intimidate
EVs: 4 HP / 252 Atk / 252 Spe
Happiness: 255
Jolly Nature
- Return
- Earthquake 
- Dragon Dance
- Protect  
Salamence is still pretty great without the mega and has Intimidate for that nice attack drop on physical attackers, which makes the team viable for X and Y too. Don’t forget to max friendship for Salamence's Return, you can just use Hondew berries + soothe bell or something. Body Slam is also a nice option if you feel like getting a Salamence from Generation 3. 
You can drop the Megas for other Mega/Strong Pokémon too, so far the only others I’ve heard have been successful are Metagross and Blaziken though.
Battle 50 only needs Meowstic, Terrakion and Whimsicott but it is safer to bring something capable of dealing with the Regitrio and Kanto Bird Trio just in case.