which are the best moves for a psychic pokemons against mew of mestireous trainer?

We'll u must have only mega horn cause it is the best bug type pokemon move there is bug is more effective than ghost and dark so just mega horn will do jut fine ok of and. Ake sure u have. A lot of ghost dark type pokemon otherwise u ain't gonna make it

Against the mysterious trainer's mew, it's suggested you get toxic, due to them being impossible to beat otherwise. Yes, you can use super effective moves, but as their levels are 10x your highest pokemon's level, it makes it extremely impossible to beat with just super effective moves.

My answer to beat mysterious trainer. Use 6x golurk/goletts. First give toxic to the mew. Then keep on giving megahorn. Like this you can beat first mew. I think there is no prize for beating mew..! So beat first mew and again restart battle and try to beat it. Then you can get max exp in creed. Hope it helped.