Unlock by beating stage 1 in any difficulty mode.
Continue playing to unlock this secret achievement.
Unlock by beating stage 2 in any difficulty mode.
Unlock by beating stage 3 in any difficulty mode.
Unlock by beating stage 4 in any difficulty mode.
Unlock by beating stage 5 in any difficulty mode.
Unlock by beating stage 6 in any difficulty mode.
Unlock by beating all stages in EASY mode.
Unlock by beating all stages in NORMAL mode.
Unlock by beating all stages in HARD mode.