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Knock out four scientists with a single swing in region 7.

The sixth level of the region is perfect for this, as it has a ton of scientists all close together. Basically what you want to do is bank your ball into the upper right area to knock out at least two scientists up there, and then one needs to fall and hit the two lower scientists to knock them out. Here's a video to show how it's done. It will likely take a few tries until enemies fall just right.