https://www.smogon.com/dex/rs/pokemon/ditto/
[Overview]
Ditto is about as useless as it gets. Why Transform into the opponent's Pokemon when you could just bring that Pokemon yourself? Ditto even needs to survive the turn to do it. The only use Ditto has is revealing your opponent's sets, but there's almost nothing slower than it that would be worth set revealing to even bring Ditto in the first place.
[SET]
name: Transform
move 1: Transform
item: Metal Powder / Quick Claw
ability: Limber
nature: Jolly
evs: 252 HP / 4 Def / 252 Spe
[SET COMMENTS]
Switch into a slow defensive Pokemon you think you can use as setup bait later in the match and scout its set. This works well with Roselia, Mawile, and Sableye that could be feasibly running a surprise set, and Roselia can even give you its Spikes.
[Checks and Counters]
Depends on the Pokemon that Ditto Transforms into.