Mewtwo is stronger than Mew generally..

There are a lot of strategies you could use, most of them are unreliable though.. You can never sure what set your opponents Mewtwo will actually have.. And on a team of 6 they'll probably have all of them..

Your only advantages will be that you can utilise sleep moves, and well.. Anyone using a team of 6 Mewtwo's probably isn't actually a very good player..

Have a nice day.