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Knock out all enemies in a level with the first swing and only with headshots in region 8.

The 12th level of the region is best for this. Lining up the shot is easy. There is a video below for reference. The hard part is getting it such that the ball actually hits the head of the first three enemies. Their hands are constantly moving up and down with those foam fingers, and if the ball hits them while their hand is up, it won't count as a headshot. The hardest part for me was getting the timing of the shot just right so that all of their hands were out of the way so it'd register as headshots. Just keep trying until you get the timing right. Here's a video: