There are 10 levels, and five different modes to play them in where you can earn a maximum of 1000 points each.
The first mode you should start with is the single player score mode. You start with five plungers. The key to scoring big is to hit rabbits with all of these shots. DO NOT RELOAD UNTIL YOU ARE OUT OF PLUNGERS. If you hit a rabbit with your first one, you'll get 100 points. The second one grants you 200 , and finally the fifth one 500. This only goes for the plungers in the same "clip", so don't spam the reload button. This will break your combo, even if you don't miss. The coding actually is so stupid that if you fire a shot and reload before the plunger hits, you will only get 100 points since the game will think that the plunger is the first one of the set. Other than that, you only need to learn where the rabbits spawn and play until you get 1000 points. Some rabbits, like the sleeping ones, the Superman rabbit and the ones with bandages around their head will grant you more points than normal. If you hit the big rabbits with headshots or hit bunnies who are really far away, you will get additional points as well. Try to stay at maximum health, since the hearts give you points when you're done.
You can play the multiplayer score mode alone. Just plug in a second controller and kill all the rabbits yourself. There are more of them in multiplayer. I got 1000 on every level in this mode on my first try this way.
This is not as easy when it comes to the multiplayer time attack. You'll probably only get 800-950 alone, so make sure that a friend helps you out if you want to score as big as possible.
The singleplayer time attack is also hard, but it's not impossible to get 1000 on the levels. 800-950 is fairly easy to get. Make sure to destroy the saucers before the Splinter Cell-rabbits jumps out . Try to hit the missiles as early as possible. Shoot the bells and music players to make the rabbits run into plain sight, dancing. This will also save you some time.
Finally you've got the Survival mode. It's just like score mode, but you only got one heart. Be careful and play like normal and you'll make it.