Cerulean Cave but you need all Kanto Badges also use a dusk ball to catch it trust me I have it
herani ha cogon.. mata2d an mo he mewto..
you can still go to cerulean cave to catch it like in firered and leafgreen
in different places it depends on what game you have (in heartgold and soul silver he is hiding in cerulean cave id use a masterball)
Cerulean Cave.
you cant
you can catch kyoger, battle red on the top of mount silver, get mewto if you haven't already, and get wi fi events
I havent played heartgold in a while but i believe the ilex forest is in other region as in not johto (btw there u can catch mewto and mew as well) 10 YR OLD OWT
mewto
you beat the elite four and then go go to the cave next to cerilion city. deep inside it you will find mewto at lvl 70. lower its health and put it to sleep or parilize it and be sure to have lots of ultra balls
You go into the cerulean cave and find your way through it, buy lots of max repels. take a Pokemon with Surf, rock climb, rock smash. when you find mewtwo weaken him until he gets red health, paralyse, sleep him and throw poke balls at him :)
you just wander around and you find MewTo
You can catch MEWTWO ,not Mewto, at the end of Cerulean cave