You are graded on six different stats. Zombie Medals are the gold medals you get from killing zombies while the platinum medals are (gasp) the platinum medals you got, usually from sparkle hunting. Sparkle Hunting is how many zombies you killed Sparkle Hunting. Completion time gives you more points the faster you finish the level and continues is how many continues you used to finish the level. The next stat is the only one that will lose you points. The final stat is only there briefly. This is the number of points you got during the level for killing zombies. You get 500 per kill and it's doubled for Sparkle Hunts. Then all of these stats are added together giving you a new final score.
As a general rule of thumb, it's more important to make the most of the zombies you kill but Sparkle Hunting frequently. Every zombie you kill (before medals collected) is worth 500 points. Every zombie you kill in Sparkle Hunt is worth 1300 points (500 per kill * 2 for Sparkle and +300 points more in the "Sparkle Hunt" category of your score card). That means that you're getting almost three times as many points as you were before. Now you need to add in the platinum medals that you get for those. For every sparkle hunt, the number of platinum medals is (Number of Zombies - 2). Each one of those is worth 400 points more. Since you have to get at least 1 for every Sparkle Hunt, you're looking at a minimum of 1450 points per Sparkle Zombie kill (+20 points because you have to get at least 1 gold medal) and that score can be higher still if you get more than three zombies in the Sparkle.
It's always easier if you see the numbers for yourself. Subtract your time bonus and then add even a 50% bonus to your score (.5 is far less than 1.3 but you have to account for other Sparkles you already got, etc. I think .5 is a very low side and good for illustrative purposes). I highly recommend that you go for better kills if you want a high score.
