(Note: multiple Mewtwos can appear, and they will have high levels. Repeat the process, but this time keep surfing along the beach of Seafoam Island and wild Mewtwos should appear. Do NOT catch him (doing so may crash your game). Surf up and down the beach until you find a Missingno.
After that, fly to Fuschia City and head from south there and surf to Seafoam Island.
When he asks you if you are in a hurry, say no and watch him catch a Weedle. To get Mewtwo before the Elite Four, talk to the man in Viridian City who teaches you to catch Pokemon.
