With a master ball. If you've already used your master ball for the legendary dog or something, tough luck. I guess you could buy about 200 ultra balls and hope for the best, but the best way is to use the master ball.
P.S make sure you have some tough Pokemon because there are a lot of tough Pokemon in this cave, though I am sure you will have no problem since you did beat the elite four =)
Edit: Before this, you have to beat the elite 4. Then make sure you have caught 60 types of pokemon and get the national pokedex from prof.oak. Then go to island 1 and celio will say you need to fix his machine. then go to mt ember (surf to the top of island 1) and go all the way to the end. After you get the ruby, then go to island 4 and find lorelei. After that, go to island 6 and go to the bottom and you will find dotted hole. Bring someone who knows cut and cut the door open. Then read the braille signs and reach the bottom and a scientist will take the sapphire away. After that go to island 5 and find the rocket warehouse. Beat the last guy (he's so easy) and get the sapphire. Lastly go to island 1 and give the stuff to celio.
Mewtwo is level 70 and is very very strong.
beat first the elite four then go to cerulean city and go to unknown dungeon. hope it helps.
this is false were the lake is by the house is obove maview city if u surf were there is 2 spaces to walk surf up and down wakk throw wall with action replay and she will battle u
You can't catch it anywhere. You'll need to trade for it from Pokemon Emerald.
either cheats or go to cerulean city and go into the cave.
You can't catch Mewtwo in Crystal. You have to trade from Red Blue or Yellow.
trade mewtwo from leaf green
No trade one from firered or leafgreen.
You can't catch Mewtwo on XD, Colosseum, Ruby, Sapphire or Emerald. Mewtwo can be caught in the Fire Red or Leaf Green Version then traded over to Emerald, Ruby, Sapphire, Colosseum or XD.
can only get mewtwo through trade in Pokemon emerald
You can only catch it in FR/LG.
you can't get mewtwo only by trading or Nintendo events
you cant catch mewtwo in Pokemon emerald. to get it you must trade with either Pokemon firered or leafgreen.
You cannot catch Mewtwo on Emerald. You have to trade for it on Fire Red or Leaf Green.
1. you cant catch mewtwo on ruby emerald or saphire<spelling mistake> you can migrate the mewtwo you catch on leafgreen or firered to migrate to platinum
putang ina tlga
Unfortunatley, you cannot catch Mewtwo in Pokemon Ruby/Saphirre/Emerald. But you can trade it in from Firered/Leafgreen. Also, instead of asking on Answers for Pokemon questions, just go to Serebii.net
You can't catch it in Emerald. T-T
You can't catch mewtwo in pearl, but you can transfer one through pal park if you have one in firered,leafgreen,ruby,sapphire, or emerald. Or you could use cheats to catch mewtwo in pearl