Having just done Flea Juggle again in my speedrun playthrough my tips are:
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Turn off the music so you can hear the sound effects clearly. I also turned off the stream I was listening to as well. As soon as I did this I performed much better. There's no clear visual cue as to which fleas still need juggled or not that your peripheral vision would catch, so you need the sound to be clear.
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When the groups of three come in, two of them are going one way and the other (most often the 2nd or 3rd flea is going the other way). You need to make sure that when you hit the one going the other way you change its direction (i.e. hit it while standing to the side, not under it). That way all three will group up and it's easy to get the 2nd and 3rd bounces. Otherwise the one that keeps going in the other direction will probably be uncatchable (if this happens, better to let it fall than risk losing both it and one of your other two).
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I used Wanderer because it's the crest I always use, but I think the fast attack animation really helps - means you have more time to walk about. I never sprinted but did occasionally dash, especially between groups.