[Arixon]

The best two Pokemon for beating dragon arena is Shiny-Giga-Dianeas with Shiny-Mega-Giratina as support. 

Stats: Shiny-Giga-Dianeas
Nature: Modest
Item: Epic Hammer
Moves:

•  Moonbeam
  
•  Magnet Rise (increases Sp.Atk, Atk, and Speed by 1)
  
•  Dragon Pulse
  
•  Aura Sphere
  

EVs:

•  HP: 252
  
•  Sp.Atk: 252
  
•  Sp.Def: 6
  

Stats: Shiny-Mega-Giratina
Nature: Relaxed
Item: Ghost Orb
Moves:

•  Protect (increases Def, Sp.Def, and Evasion by 1)
  
•  Shadow Ball (important for decreasing enemy's Sp.Def)
  
•  Toxic or Shadow Claw
  
•  Dragon Pulse
  

EVs: 

•  Def: 252
  
•  Sp.Def: 252
  
•  HP: 6