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. Before you can unlock it's evolved form you must first purchase the Guardian Bakugan and when you then defeat it for a second time their evolved form of their Guardian Bakugan will become unlocked.
Dan: Pyrus Dragonoid evolves into Pyrus Delta Dragonoid II
Runo: Haos Tigrerra evolves into Haos Blade Tigrerra
Choji: Aquos Preyas evolves into Aquos Preyas II Angelo/Diablo
Julie: Subterra Gorem evolves into Subterra Hammer Gorem
Shun: Ventus Skyress evolves into Ventus Storm Skyress
Masquerade: Darkus Hydranoid evolves into Darkus Blade Hydranoid
Marduk: Darkus Vladitor evolves into Darkus Battle Axe Vladitor