For the Insane Juggler achievement, I was able to do that in the Boss Battle area for Chapter 5 of the Fight Lab on Bob I think it was. At a certain point, they will fall down and stay down, telling you to finish them off with the Bound/Tag move. If you can do that move, and keep your partner on the screen with you for a little bit, you are able to continue attacking them while they are in the air. The enemies get larger with the less health they have left, and thus are easier to continue hitting.
I'm sorry I cannot be more specific in how to do it, but I am not very good at this game. I know that this IS where I got it, and I noticed what happened while I got it. Hopefully someone can fill out the details in order to help out, and make this a good strategy (unless there is a much easier way of doing it somewhere else).