Hört sich spannend an:
The goal of the game is to complete all the rooms successfully. The only way you can complete a room is to have 0 or positive points when you have tilted over all the red/green boxes.
Normally you cannot complete a room just by shooting at the boxes directly. You will get too many penalty points for using too many bullets. Instead you need to use the score multiplier feature. This can be done by bouncing the bullet from the wall before hitting the red/green boxes. This way the points you get will be multiplied by 10 (or 20 or 30, depending on how many bounces the bullet makes before hitting the boxes).
Note that the first shot is always free, regardless of weapon. A good way is to use either the shotgun or the bazooka for the first shot, since they are much more powerful that the revolver. After the first bullet you probably want to use the revolver, because you get much less penalty points for using the revolver bullets.