Well you would think on Dialga but no, you should use it on Mespirit because when you try to catch Mespirit it keeps running away from you so you mite as well get it on your first try.
Chat with our AI personalities
well you can use it on anyone you want just not a trainer i would suggest giratina
if u use any masterball cheat u get bad egg in Pokemon PC
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!
You should use it on one of the 'roaming' pokemon, like Cresselia in Diamond/Pearl/Platinum.
You probally want to use it on mewtwo because it is level 70