A juggle is when you hit an airborne opponent! For this combo, you'll have to hit the same enemy while they're airborne ten times before they hit the ground! This might sound difficult, but given how most hits tend to knock your opponent upwards just a little, you shouldn't have too much trouble. Especially with the air-based moves, although you really won't need these!Now you could try to come up with a convoluted way to get ten hits on an airborne opponent, but the best and easiest way to do it is shown below. Note that you can do this on many levels, not just the one shown!

Best Method for And Stay Up There

Alternatively, with decent timing, you can juggle an enemy infinitely by walking back and forth under them as they fly through the air. You can do this anywhere as long as there's nothing to stop you (like an enemy trying to attack!), so go until you get tired! If you're having trouble with  To the Top, this infinite combo can get you there as well. Here's a reference:

Rita's Rewind Max Combo Counter!