Most people, it would seem, use it on Dialga (or Palkia in Pearl). It isn't necessary; unlike in other games, there isn't a single Pokemon that you must have it to catch.
I would suggest saving it for Mesprit or Cresselia if you don't want a big hassle chasing them down over and over, but they are quite simple to catch otherwise.
Giratina would be a good candidate, as it is extremely hard to catch (though I caught mine with a Timer Ball after quite a long battle).
It really depends on whether you mind a long battle or not. I caught Mesprit and Cresselia with very little fuss using Ultra Balls, but Giratina took a long time. If you don't mind hunting down Mesprit and Cresselia, I'd save it for Giratina.
Or in my opinion you could use it on Darkrai as it is very hard to catch with any other ball (even though i the Pokemon master caught it with a pokeball)
After defeating Cyrus in Veilstone, he should give a Master Ball to you.
giratina
Master Ball
From what it looks like now, you can't get more then 1 Master Ball in any Pokemon game. However, you can trade them over to another Pokemon game. Example : If you need a Master Ball on Platinum, you can still trade it over from Diamond and/or Pearl, or even from HeartGold and SoulSilver. I'm afraid that's the only option to get more then 1 Master Ball.
You cannot find a second master ball however you can trade Pokemon to get another Pokemon with a master ball or use an action replay
no
probaby kyogre
yes u can in all pokemon games
You cant
profecer elm givs it 2 u
The trick is to save your master ball.
To get an additional master ball, you must either trade it from another Pokemon game, or use cheats to get one.
one of the monks in the bell toweer will give you it
You have to fight it until its weak then use a master ball
I sugest using master ball... that's how i caught it
After you beat Lance, go back to professor Elm and talk to him. Then, he'll give you the master ball! (You can go to pokemon.marriland.com.)
1. start a new game then trade it from that game 2. Mirgrate a Pokemon holding it