How to: Defeating the Elite Four

One singular level 100 Mewtwo

Moves: Shadow Ball, Aura Sphere, Calm Mind, and Psystrike

EVs: 252 Special Attack, 252 Speed, 4 Special Defense

IVS: Max IVS using Pokeimproves

Items: About 30 full restores (Just to be safe) and 3 max elixirs

Lorelei: Use 2 Calm Minds then Aura Sphere her Pokemon (If her Dewgong uses Toxic, wait until her Mamoswine to use a full restore, as the Dewgong and Walrein will most likely use more Toxic on you)

WARNING: You can waste a full restore before fighting Bruno, but it isn't required.

Bruno: Use 2 Calm Minds then Psystrike his Pokemon EXCEPT for his Gallade, for that use Shadow Ball (If his Machamp uses Fire Punch and it burns you be sure to heal before you move on, as the following Pokemon will likely deal more damage)

WARNING: You MUST use a full restore before challenging Agatha, or you will most likely lose.

Agatha: Set up 3 Calm Minds and Shadow Ball up until her Gengar, use Shadow Ball on it and her Giratina.

WARNING: Use a Full Restore and a Max Elixir before challenging Lance.

Lance: Set up 3 Calm Minds and Psystrike all of his Pokemon except for his Dialga, who you have to use 2 Shadow Balls on.

WARNING: Use a Full Restore and Max Elixir before challenging Gary

Champ Gary: Set up 3 Calm Minds while healing as his Metagross can K.O the Mewtwo if it gets an attack boost from Meteor Mash. Then use: Shadow Ball on Metagross, Aura Sphere for Tyranitar, Shadow Ball for Alakazam, Aura Sphere for Slaking, Psystrike for Garchomp, and 2 Aura Spheres while healing with Full Restores for Arceus. And that's it! You've beaten the Elite Four!
 * Make sure you calm mind as fast as you can, cause if Metagross gets an boost, your goner because of the boost. *