You probally want to use it on mewtwo because it is level 70
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you get it when you beat all the johto badges in the game and prof. oak will call you and give you a masterball p.s never let a sister use your pokemon games if she doesn't know pokemon and you have a masterball hope this helps!
if u use any masterball cheat u get bad egg in Pokemon PC
No
use headbutt on trees
A masterball would be the best choice but if you want to save it, you should use a Pokemon that does not let him flee (he always flees after the first turn) and use regular pokeballs or ultra balls