sounds much less grindy than "clear <game mode> with the entire cast" and more challenging than "get x kills in cruel smash using whoever you want, so feel free to pick the character with the best tools for cheesing it"
at any rate I tried it on 3ds for giggles and got it after half an hour. @
Kofu
I think the best strategy is to hang around near the ledge until they all cluster up, then roll from the ledge>fsmash them all off>gimp with fair as needed (Miis don't upB or grab the ledge, but they always footstool/walljump if able; something to keep in mind). @
Nammy12
's suggestion is great for singular Miis if they don't cooperate (throw invuln actually saved me for KO #7), but you can fsmash four off at once and it speeds things up quite a bit to kill them en masse.
Lucina's usmash under the side platforms works amusingly well for getting them herded at the start too, though I'm not sure if it really contributed anything overall.