Finally did it yesterday, but I had to change up my strategy a little.
I still utilized GameXplain's method, but instead of staying under the platform the whole time, I'd do the initial recovery to knock the Miis back to the middle of the stage, jump up from the ledge if the coast was clear, and hang close to it midair to entice to Miis to come towards the precipice, air dodging on the way back down.
Now that several Miis are clustered at the very edge, repeat the first step and one, two, or even three Miis will end up swiped of the stage, but be aware they will footstool jump each other or even you to recover back, so be very careful.
A risky technique, and one that required several tries, but if done right the challenge can be completed rather quickly.