Easy strat for platinum, maybe even max if you have a good aim like me.
Note that there are 17 cut outs that you have to hit with a maximum score of 100 per cut out giving a total score of 1700.
Platinum score is 1250 and so should be easy. It's all a matter of learning the pattern.
OK here is what I do, feel free to adapt it to your own style though.

1. Use the sniper rifle at all times.
2. When the starts run right up to the window.
3. Put up the sight and slightly to your right on top of the blue crate the first cut out should appear.
4. Aim for the shorts on this person.
5. Next person appears down and to the left of the first.
6. Aim for the top of the right leg.
7. Quickly aim directly below you in the middle and get the third cut out in the butt cheek.
8. Now for some quickness or time taking just adapt to the following.
9. Remove the sight and run to the back right corner of the room.
10. A cut out should appear over the window.
11. Again aim for her backside.
12. Now for some speed. Run to the window and aim right. Shoot the demon in the groin.
13. If you miss the demon don't worry you have a chance to get him later.
14. Aim down and to the right of the blue crate that the first cut out appeared on.
15. A cut out should be appearing from the right hand side.
16. Aim to hit her in the thigh.
17. Now aim down and left to the grey wall.
18. There should be someone here, aim to hit him in the shorts.
19. Now aim for the far right corner and again shoot this person in the buttock.
20. Directly in front of you on the left hand side of the crate the next cut out should rear its head.
21. Shoot the demon in the groin.
22. Aim down a bit more and to the left.
23. A moving cut out should appear, shoot her in the butt.
24. Now to make up for the miss earlier (assuming you missed).
25. ASAP aim up and right, taking out the demon and then the woman who appears next to it.
26. Next one appears right at the back, on the ground to the left of the boxes.
27. Stand on the left hand side of the window and look to the middle right.
28. A man appears here, shoot him quickly.
29. Now swing over to the right hand side of the window and look way off to the left
30. A target appears here and is almost side on but it isn't to hard to get 100pts on
31. Now comes the hard bit. ASAP get to the next target as described below
32. Now you need to aim right into the bottom right corner.
33. Now straight ahead but up high on top of the crates in the distance.
34. Now aim down again at the grating slightly to your left, shoot the final person quickly before the timer runs out.
35. Sit back and admire your score of at least 1250-1500.