Question

Like do they have to have certain natures, are certain Pokemon banned, etc.

Answer 1

• All Pokemon with Metronome are allowed. No bans on Pokemon. Even Mewtwo.
• Leppa Berries are the recommended item.
• There are no nature bans of any sort.
• EVs can be placed anywhere, as implyed by the sources "defensive Mewtwo".

Source

Comments

I wish Showdown had this tier.