Pokémon Stadium/Mewtwo

How To Access
In order to battle, you first need to get all 10 trophies in the Stadium Mode as well as the Gym Leader Castle.

Once you have done that, the VS Mewtwo bonus tournament will open up on the map. To access it, simply move your Magne-cursor up to the top. Unlike the other battles in the Stadium and Gym cups, the VS Mewtwo tournament allows you to enter six Pokémon.

Mewtwo's Synopsis
Before battling Mewtwo, know everything about it:

Stats:
 * Type: Psychic
 * Level: 100
 * HP: 322
 * Attack: 202
 * Defense: 166
 * Special: 281
 * Speed: 238

Moves:
 * 1: Psychic (Psychic). 135 Damage (90 x 1.5x). 100% Accuracy.
 * 2: Blizzard (Ice). 120 Damage. 90% Accuracy.
 * 3: Rest. Sleeps for 2 turns that recovers all health, Paralysis, Poison, and Burn.
 * 4 (R1 only): Thunderbolt (Electric). 95 Damage. 100% Accuracy.
 * 4 (R2 only): Amnesia. Increases Special Attack and Defense by 100%. Caps at 400% after 3 uses.

Choosing Pokemon
Using the Transfer Pak from Red, Blue, or Yellow, will make this battle much easier due to acquiring EVs (Effort Values) that increase due to items or the number of battles each Pokemon does. In addition, you can bring your own Mewtwo to make this fight much easier.

Otherwise, if you are relying on rental Pokemon, don't worry. There are plenty of Pokemon to choose from.

Choose your 6 Pokemon wisely. Electrode should be chosen first because it is the only Pokemon that is faster than Mewtwo and has 2 key moves: Thunder Wave and Flash. As for the other 5, here are some key notes:
 * Pokemon weak to Psychic, Ice, and Electric (Round 1 only) aren't recommended due to getting KO'd in 1 hit unless that Pokemon has very high special like Slowbro.
 * Pokemon with very low HP and/or Special are not recommended due to getting KO'd in 1 hit.
 * You can use One-Hit KO moves like Horn Drill to easily beat Mewtwo. Mewtwo is not immune to this.
 * Mewtwo cannot switch out in battle. Statuses like Leech Seed, Confused, and Lowered stats like accuracy will still remain intact. Rest will not remedy these statuses.

Battling Mewtwo
Start the battle with Electrode. Get that Thunder Wave to paralyze Mewtwo, which will cut his speed in 1/2 and have a 1/4th chance of being stunned. If you are very unlucky to have Mewtwo's Psychic (which it will always use against Electrode) critical hit your Electrode and KO it, don't worry too much as you can still beat it. If Electrode is alive, spam Flash until it gets KO'd to lower its accuracy.

After that, you can choose any of the paths to choose from:
 * 1. The One-Hit KO method. Pick 5 Pokemon who has a Fissure, Horn Drill, or Guillotine. Spam until it hits Mewtwo. Retry battle if it doesn't.
 * 2. Lowering accuracy and defense. First, spam Flash until it is at its lowest accuracy (6 times). Spam Screech 3 times. Have Pokemon that uses heavy damaging physical moves to defeat Mewtwo in 1-2 hits.
 * 3. Confuse and Leech Seed. Having both confused and paralyzed will make Mewtwo's job a lot harder. Leech Seed is just there to recover. You can mix this with number 2 above.
 * 3. Use Ditto. Though Mewtwo resists Psychic, spamming Psychic has chances of lowering its Special stat. If it used Rest and wakes up, you can spam Blizzard in hopes of freezing. For Round 2, using Amnesia 3 times will make Mewtwo defeating Ditto much harder. The only things to worry about is being unlucky of frozen and paralysis status.

If Mewtwo gets into Yellow or Red on health, it will use Rest. You have 2-3 turns to KO it.