it's not necessary to strike up to juggle them. you can do a regular strike sideways. i didn't notice it from the beginning, but it did help me a lot completing the challenge. i also didn't jump too high and mostly ran sideways sweeping the ground when there are a lot of fleas, and especially when they happened to land far away from each other. but of course, try to "group them" and send same direction as best as you can.