Ok, first you have to beat the Elite 4. If you already have the ruby and sapphire and you gave them to Celio then that's great. But if you don't then unfortunately I won't include it in this because is a lengthy process.
Go to cerulean cave. Accesible by Surfing on the water. When you get inside it is a HUGE puzzle so I am going to reffer you to:
http://www.YouTube.com/watch?v=3T4rydSKPfU
Just two more sentences of wizdom...
SAVE SAVE SAVE before!
donot use the Master Ball
Have it traded to you from a Firered/Leafgreen.
get the ruby and the Sapphire gems from 4and 6 islands . the go to ceroolien cave.you can find mew2 in there.
you can't only mew2 there are a bunch of myths but they aren't true
zapdos, moltres, articuno, mew2, and all Legendary Pokemon in Gold/Silver/Crystal and Ruby/Sapphire/Emerald but only catchable ones are the first four listed.
mew2 is the rarest
from a gba game.
palpark mountain parts
well just beat them 12 times then go to the cave where mew2 is but then at the place where mew2 is mew is there by blake.omostyle
get every Pokemon in the game then go to mystery cave and catch it
Pokemon The First Movie Mew2 Strikes back
Yes you can trade leafgreen version Pokemon to another leafgreen version.
U have to have a ancient kanto artifact open it and it has to be mews dna then go to the labatoy by the normal area and us it and mew2 will appear