It's all about camping level 3. Each of the green things that Sark shoots are worth 600 points. If you can get both of them when they split, obviously you'll be racking up 1,200 points each time.
Just keep bouncing from platform to platform, shooting discs and waiting to kill spinners and those green blobs. Just don't kill Sark!
I practiced about an hour a day for 4-5 days until I finally got the hang of it and now I could probably play forever without losing a life.