Win the Ultimate Battle Tournament, and defeat Marduk in the final battle. Then, 
go to the Park, and challenge any of the following characters to unlock their 
Guardian Bakugan at the Bakugan Store. Defeat them a second time to unlock the 
evolved form of their Guardian Bakugan. Note: You must first purchase the 
Guardian Bakugan before you can unlock its evolved form.

  Choji: Aquos Preyas; Aquos Preyas II Angelo/Diablo 
  Dan: Pyrus Dragonoid; Pyrus Delta Dragonoid II 
  Julie: Subterra Gorem; Subterra Hammer Gorem 
  Marduk: Darkus Vladitor; Darkus Battle Axe Vladitor 
  Masquerade: Darkus Hydranoid; Darkus Blade Hydranoid 
  Runo: Haos Tigrerra; Haos Blade Tigrerra 
  Shun: Ventus Skyress; Ventus Storm Skyress