Win the Ultimate Battle Tournament and Defeat Marduk in the final battle. After that, go to the park and challenge any of the following characters to unlock their guardian Bakugan at the Bakugan Store. Defeat any of 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 Ax Vladitor
Masquerade - Darkus Hydranoid; Darkus Blade Hydranoid
Runo - Haos Tigrerra; Haos Blade Tigrerra
Shun - Ventus Skyress; Ventus Storm Skyress