The team:
 
G (Gengar) (G) @ Cleanse Tag 
Ability: Levitate
Level: 50  
EVs: 212 Atk / 116 SpA / 180 Spe  
Mild nature  
IVs: 31 HP / 31 Atk / 31 Def / 31 SpA / 31 SpD / 31 Spe  
- Psychic
- Thunderbolt
- Ice Punch
- Explosion

H (Heracross) (F) @ Choice Band
Ability: Guts  
Level: 50  
EVs: 252 Atk / 4 SpD / 252 Spe  
Adamant nature  
IVs: 31 HP / 31 Atk / 31 Def / 31 SpA / 31 SpD / 31 Spe  
- Megahorn
- Brick Break  
- Facade  
- Earthquake
 
L (Latios) @ Lum Berry
Ability: Levitate
Level: 50  
EVs: 4 HP / 252 SpA / 252 Spe  
Modest nature  
IVs: 31 HP / 0 Atk / 31 Def / 31 SpA / 31 SpD / 31 Spe  
- Psychic  
- Thunderbolt 
- Surf
- Dragon Claw

Notes:
- Double battles can't appear if you have 1 Pokemon left. So if you get an early double battle, explode with Gengar for a quick win that also prevents future double battles. The Cleanse Tag still works even if Gengar is fainted.
- Heracross is ridiculously powerful with Guts + Facade + poison/burn from one of the status rooms.