I think the "laziest" way to do this is to get two Lucario's
and use his neutral B(Link) with both of them beside each other.
It's advised you stay at the screen though, because if a bomb drops or one of the Lucario gets hit, you'll have to redo it again.
This is really easy and lazy way, and you still beat the challenge(even if you used 2 players).
Also, I wasn't clear but you don't actually have to have two people to do this.
All you need is just 2 controllers. You'll still get the reward even if you do it with 2 players, though, if one player is KOed, it's gameover.
Anyway, get 2 controllers. Technically, you're not controlling 2 characters at once, so this isn't too complicated, just make sure you do this fast.
Quickly, move both Lucario to the middle platform then make them face the Opposite direction(NOT FACING each other).
Move one Lucario to the middle, use Neutral B.
Move the other Lucario, use Neutral B.
Make sure both Lucario's ARE NOT facing each other, and are facing opposite directions. Make sure they're close to each other, not too close though.
There, you're done. Usually, if any alloy gets nearby, they'll get locked into the Neutral B, and they won't get knocked back even at 999% UNLESS an item does it for you.
In any case, you won't really have to do much now. Just wait but make sure you keep paying attention to the game and just don't go AFK.
Things to note:
1. Careful of bomb or item drops. This will probably be the thing that screws you up.
2. Though, this is probably the laziest-easiest way to do it, make sure you keep watching the screen though.
If one Lucario gets interrupted, reposition them at the center platform again and redo the Neutral B.
3. The reason why you want to do this on the top middle platform is because alloys spawn near it or on it.
They'll spawn, then immediately get locked into the Lucarios' Neutral Bs. If they don't get knocked in, they will eventually but they MAY get an attack off, so make sure you pay attention to the game and don't go AFK>
