Catch Mewtwo Without Beating the Elite Four

Want to catch Mewtwo on Red/Blue without using GameShark or beating the Elite Four? This glitch will help.

Things You Need

A Game Boy,
Pokémon Red or Blue,
- Access to Lavender Town,
- A Pokémon that knows the HM Fly,
- To not have beaten the Gambler on the Route west of Lavender Town,
- A Pokémon with a Special stat of 131.

Go to the Route west of Lavender Town, and save just in case.
Walk up to the Gambler standing at the end of the Route, but immediately press Start while stepping.
Choose to fly away to a Route where there is a trainer you have not beaten yet.
Walk up to the trainer and you have to win; otherwise the cheat will break. Also, make sure there is at least one space between you and the trainer when he notices you, or the game will freeze.
Do not go in any tall grass after you beat that trainer! Now, fly to a place where you can find some wild Ditto.
Once you find Ditto, switch to the Pokémon with a special stat of 131. Beat the Ditto.
Fly back to Lavender Town and head back to the Route with the Gambler.
The menu screen will pop up. Press B to close it.
Suddenly, a Mewtwo will appear. Catch it, and congrats! You caught Mewtwo!

Using a Psychic type of your own, is effective as Psychic resists itself, this works better if you have a move that hits Mewtwo for neutral damage.
Mewtwo is Psychic type, so use Water type moves. This is because Water has a normal effect, it doesn't cause status ailments and Psychic-types don't have a type advantage.
Have a low-level Pokémon in your party so you can catch the low-level Mewtwo.