Sorry didn't see until now, but toss. Unfortunately, it's not a matter of if, because Defog and Soft-Boiled are transfer only, therefore not an option. But defensive sets still work well with Will-o-Wisp, Taunt, Knock Off, or the many other support moves that it has access to. And you can have Roost. Will-o-Wisp is the premier status move of choice though because it can cripple Pursuit users, which it hates. But it's not as though Mew has no other options, but being "only" 100 base stats across the board but pure Psychic hampers it in many roles that other Pokemon can do better, including its other fellow base 100 all around friends.