He does not launch the shield. He makes it grow larger and larger until it reaches the limit that he wish for. In this case, the walls of effect at 5:43.
A bad soup can kill you, that does not mean it is all powerful.
Mew did not even want to fight him then. All Mew did was avoid attack after attack, taking it all as one big game. He did not even put any effort into it, if you look closely. Mewtwo himself knows and says that Mew is not putting any effort into it.
Remember that Mewtwo is an enhanced version of Mew 🙂
(And he was hit by an energy bolt)