if you still have a master ball you can catch it but you can also save you masterball for the three dogs (entie, raikou and scucine) or ho-oh and lugia.
if you want help catchin mewtwo reply
um..its at level 70, not 80.....
Well assuming that you have completed the game up to the point where you can enter Kanto. Which is only accessible if you beat the Elite Four for the 1st time. Then you will Enter Viridian City the 1st City in Kanto Region that you can interact in. Go South to Pallet Town and Get the updated Pokedex from Professor Oak in his lab then continue on the quest like you were playing one of the 1st Generation Games Cerulean City should be just after Mt Moon (dont worry it is REALLY small when compared to Mt Moon in 1st Gen games).
Cerulean cave
Cerulean cave
you catch it in cerulean cave
The Cerulean Cave
You catch Mew-Two
mewtwo is in cerulean cave. the cave in in cerulean city, where misty's gym is. mewtwo in hard to catch, though.
If you mean, "Wobbuffet", you can catch it in Cerulean Cave.
you can catch mewtwo at cerulean cave
Lot a thum in Cerulean Cave.
go to the cerulean cave and catch mewtwo
cerulean cave works but its strong.
Cerulean Cave, Deepest point.