Are you hav'n trouble getting a picture of Mew?
Well then I'm the 1 u need to look 2. All you have to do is wait for Mew 2 come.
Then throw pester balls and/or apples at him. Then his pattern will change. He'll start coming towards you.
While he's doing this, throw more balls and apples at him. Then it'll look like he's racin' u, But this time he won't be in his guardball anymore.
I hope I've helped you finish the game(BECAUSE THAT'S WHAT HAPPENS WHEN YOU GET A PICTURE OF MEW).