The key to get a high score in this game is to think outside the box. The given solutions aren't always the best. For instance level 24, you can complete the level by immediately shooting the first zombie in the legs, then shoot the glass and with a little bit of luck you finished the level. Level 39: don't shoot the right platform, shoot the zombie with a cannonball instead. Saves you 3 shots. These kind of ''errors'' allow you to get a high score.
I thought the game was really hard, because I can't control very well with my left hand. So I decided to let all the enemies live and just jump over them, so I can control the arrow keys with my right hand. It sounds stupid, but I got over 1000 now instead of 200.