Mew is the hardest pokemon that i seen but i know how to get him all you need is a music box (off course) ,you need 1 pokemon lvl 100 only up too three pokemon in your party. Lucario rank which is the best way to get a legendary, and friend bow which is another best way to get a lengedary.

Ps: this is not going to get you mew that fast you need too wait to you know