First, you have to go to the Underground passage to Lavander then when you come out step as far away from the trainer that's staring at you with Pikachu still on the door then use Fly and go to Curulian city, but make sure that his exlmation point is up, but also make sure that he dosen't battle you or it will completlelly messed up! When you get to Curulian go on the path with the five trainers then go to the trainer in the middlenorth that says this " I had a party on the S.S Anne with my dad " or simply the one with the Slowpoke, this can also mess the secret of getting Mew. Then after battleing him Fly to Lavander town. Then go on the short path before going on the Rt. to Cleadon Just go 1 step then Save in that exact spot, then this light appears and then Mew will attack ( the cra-y part is that its only on Lv.7 )
I sware this is the exact way to get Mew!