Pokémon Stadium/Mewtwo

In order to battle, you first need to get all 10 trophies in the Stadium Mode as well as the Gym Leader Castle. Once completed, 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
Stats:
 * Type: Psychic
 * Level: 100
 * HP: 385 (R1); 415 (R2)
 * Attack: 276 (R1); 350 (R2)
 * Defense: 236 (R1); 306 (R2)
 * Special: 364 (R1); 447 (R2)
 * Speed: 316 (R1); 394 (R2)

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

Choosing Pokémon
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 Pokémon does. In addition, you can bring your own Mewtwo to make this fight much easier.

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

Choose your 6 Pokémon wisely. Here are some key notes:
 * For Round 1, Electrode would be the most preferred choice and chosen first because it is the only Pokémon that is faster than Mewtwo and has 2 key moves: Thunder Wave and Flash.
 * For Round 2, no rental Pokémon is faster than Mewtwo. Electrode can still work, but pray that Mewtwo does not use Psychic and critical hit it. Using any other Pokémon with Thunder Wave and can take a Psychic or Ice can work as well.
 * Using a 2nd Pokémon with Thunder Wave, in case Electrode failed its job by an unlucky chance, can still work.
 * Pokémon weak to Psychic, Ice, and Electric (Round 1 only) aren't recommended due to getting KO'd in 1 hit unless that Pokémon has very high special like Slowbro.
 * Pokémon 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 and is very preferred.
 * Mewtwo cannot switch out in battle. Statuses like Leech Seed, Confused, and Lowered stats like accuracy (Flash) and defense (Screech) will still remain intact. Rest will not remedy these statuses.
 * Even though Psychic is weak to Bug moves, this generation does not have any useful Bug moves. Leech Life does pitiful 20 Damage and Pin Missile does 14 Damage (up to 5 times at 12.5% chance). It doesn't help that all of the Pokémon who can learn either of the 2 moves either have weak attack or are weak to Psychic or Blizzard.

Battling Mewtwo
For Round 1, 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.

For Round 2, Electrode can still work, but pray that Mewtwo does not use Psychic and critical hit it. You might need to prepare a 2nd Pokémon with Thunder Wave in case Electrode failed at that worse chance possible or restart. If you chose a different starting Pokémon outside of Electrode due to being able to take a Psychic or Blizzard, the same Thunder Wave strategy works as well.

After that, you can choose any of the paths to choose from:
 * 1) The One-Hit KO method. Pick 5 Pokémon 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 Pokémon that uses heavy damaging physical moves to defeat Mewtwo in 1-2 hits. Very risky yet powerful moves like like SelfDestruct and Explosion will do wonders.
 * 3) Confuse Ray 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.
 * 4) 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 (Round 1 only) status.

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

If you are using Transfer Pak, use Light Screen Chansey, Swords Dance Mew, Exeggutor, Tobybro, your Own Mewtwo and Self Destruct Snorlax to make quick work of this battle.