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Fast Milestones

Easy milestones if you set stocks really high, use all player-controlled characters, 0.5 knockback, and no time limit.
Knockback scaling needs to be set to 0.5 so each time you knock a player off the screen, it counts as 2 kills.

Use the up smash attack to know the three other player-controlled characters off the screen. If you follow the above method, each one knocked out counts as 2 kills towards the milestones.

Use this before they patch this method out like they have with other methods.