mewtwo

The game registers hours played per person, so if you play a five minute match with four human players, it registers a twenty minute match. Your best option is to put in four controllers and set your game to a 1-Stock Match. Then turn off items. Pick a simple stage with no moving platforms or obstacles that can not knock you off. Check the clock and start the match. Five hours later, you should have twenty hours of gameplay. Just jump off the stage with three of the characters, and you should be able to challenge Mewtwo.

Combined VS play time is the amount of hours played by each person in total. For example, if two players play the game for two hours on VS mode, four hours will be added to the combined VS play time. Three people playing for one and a half hours will add four and a half hours to the combined VS play time. See where I'm heading with this?

Anyway, this proves that Mewtwo can be unlocked after playing for five hours, if you have four controllers. So, just play VS mode with three friends for five hours (or less for a longer period of time). Or just start a stock match with four players, turn the TV but not the Gamecube off, go and do something for five hours, and come back five hours later. Have whichever character you want win the match, and then Mewtwo should challenge you.

It is true.
So basically saying saying you play with four people,your total of VS playtime is quadrupled. So pick any kind of stage that doesn't involve moving platforms. Wait 5 hours,then let 3 people kill themselves with only 1-Stock and you play against Mewtwo just within 5 hours. Hope this helps you out.

Play as Mewtwo
Mewtwo will appear at random at the end of a multi-player match and challenge the winner. He will be unlocked as a selectable character after he is defeated. Alternately, play 20 hours in melee vs. Mode (combined number of human players times the number of hours). For example, if you play 1 human player vs. 3 CPU players, play 20 hours; 2 players vs. 2 CPU players, play 10 hours; 3 players vs. 1 CPU player, 7 hours; 4 player vs., 5 hours. To get Mewtwo easily, go into Custom Rules Melee and change "Stock" to "3" and "Item Flow" to "None", and "Stock Time Limit" to "Off". Choose two of your best characters (for fighting Mewtwo) and go into a match in the Pokemon Stadium level. Place both fighters on the Pokeball in the middle. Do not pause the game. Unplug the controllers, and leave the system on overnight to accumulate game time. Plug in the controllers again, and have one of the fighters die three times. If enough hours have been accumulated, Mewtwo will challenge you. Defeat him to unlock him as a playable character