You, Joey, and Snorlax (with Maruto) enter Saffron City to find a ghost protection wall in the way. A Haunter grabs Joey and takes him away somewhere. Meowth enters the scene and tells the ghost Pokémon to let you through. Snorlax leaves to get his promised snacks as Meowth introduces himself as the human-to-Pokémon translator in the war against Mewthree. You and Meowth take Maruto to the place where a doctor lives , where a fake Maruto reveals himself to be Sabrina, who is going to take over the city. She also reveals that she has Mewtwo, captured for the enemy's side. After beating the level, you will be able to use the TM Hydro Pump.
No rare or difficult-to-catch Pokémon are used in this strategy. The Pokémon should optimally be level 64, but this strategy will work even with a quite lower level Pokémon sextet. Mewtwo will faint even before touching Meowth.
All you need is:
-Three Bulldoze users
-A Roar or Whirlwind user
-A Helping Hand user
-A pokemon with high damage to help deal with mewtwo - not needed if you have got your Pokémon on level 64.
Where to put them: On the top three spots, put the Bulldoze users. At the side, still on top, you can leave the Helping Hand user; at his right side, place the Earthquake user (using a different move, preferably a strong Fighting-type one).
Done. You won’t need to move a muscle, except that you preferably have to keep the Pokémon buffed up with the Helping Hand (it is just semi-mandatory when Mewtwo appears). Also, if you have weaker Pokémon and need to guarantee that the “Roarer” won’t be damaged by the Poison from the Venomoth, you can click on him (thus restarting him in his position and taking the Poison away).