I was able to beat it by sheer luck. I landed a dude on the little shelf thing on the left, dropped the big green ball, and it just so happened that the star touched the guy on the shelf on its way down. I haven't been able to do it again, and I'm sure it's a fluke. I also had to shoot about five or six guys to get one to stay on that little ledge.
I've also found that, if score is unimportant, you can launch a bunch of guys at the "top shelf" on the left, and if you hit it just right, they'll shoot up into the area with the star and two small green balls. If you keep doing it, you can knock the star out, and glitch a guy through the green lever if you just keep launching at full power. Average score of like maybe 200 that way.
But I'm hoping someone knows of a way to do it and maintain some sort of respectable score. This is one of the few where I don't have a score at or over 800.