ohhhh right that was definitely me, I remember this conversation.
The idea was that Mew is usually seen as a Nasty Plot sweeper or a Will o Wisp/Taunt/Roost staller. So instead, I would use Swords Dance. Most people would switch in a snorlax or some special wall expecting to just stall out the mew. Then Mew can use Drain Punch and KO, or Zen Headbutt if they're not weak to fighting. After Mew takes out the special wall, Empoleon would use Substitute and Agility while holding a Petaya berry to set up a sweep, because there would be no special walls standing in its way. If you read my UU RMT titled "Ecstatic Waters" it works the same way when I used a Mew + Yanmega core.