What you need:
Masterball x 1
revives x 10 or more
Full Restores x 10 or more
Step 1: Go in Cerulean Cave and catch Mewtwo with your Masterball.
Step 2: Train it a couple of levels
Step 3: Teach it these exact moves: Rock Slide, ThunderBolt, Strength and Psychic
Now make your way up to Red but don't fight wild Pokemon with Mewtwo unless you have enough ethers
once you reach Red, save and fight him. On his Pikachu use Psychic and that should take him out or in a few more hits.
For his Blastoise, use ThunderBolt same for his Lapras.
For his Venusaur use Psychic.
for his Charizard use Rock Slide
for his Snorlax, use basically anything
Remember! if Mewtwo faints use your revive and if your pokemon survives a hit use a full restore on him. THEN if your pokemon survives for a 3rd turn just attack you've got nothing else to do.
I beat Red with a level 73 Mewtwo and a little help from level 50s