Space Invaders Extreme (DS)
Complete a stage in Arcade mode under Normal setting to unlock it at the "Extra" menu. You can then replay that stage as desired.
Complete the game in Arcade mode and all five stages in Stage mode. There are no Bosses in this mode, with the stages cycling endlessly.
Successfully complete Arcade mode. Extra mode will be available after Arcade mode is selected again. It features a higher difficulty.
Strategy guides from GameFAQs