Juggle Hits are attacks which hit an opponent while they are in the air. You can usually launch an opponent with a medium or heavy special, then attack them with some other move to add to the match's total.
For the achievement, you do not need to get all hits in a single combo, just across the entire match. An easy route to this would be to set up a versus match against a Noob CPU. I used Spinal: his Soul Sword opener can launch in certain situations, while a Shadow Searing Skull can hit a launched enemy multiple times while they are juggled.
Hits made during an Ultra also count toward the match tally; Spinal's Ultra ends with a series of attacks which launch and juggle the opponent. You should be able to get around 10 hits from this alone.
The challenge will unlock once you hit the Match Results screen. If you need more practice with juggling, check out Dojo Advanced Lesson 30.