no, you use an ultra ball on Mesprit and quick balls on Cresselia.
it would help to train a golbat to about lv. 50-55 (depending on stats) and teach it mean look. then start chipping away at there hp since they won't be able to escape. then do what you do for any legendary. another thing you can do is train a trapinch (migrate from hoen) with arena trap as its ability. but to be safe don't use timer balls the effect only works on non legends and ainchents.
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You should use it on one of the 'roaming' pokemon, like Cresselia in Diamond/Pearl/Platinum.
A Master Ball
Yes. Right after you defeat Galactic Boss Cyrus once. I recommend you don't use it on Palkia, though, as there are much harder legendary Pokemon after the game. Use it on Mesprit or Cresselia later on.
Well,mostly on Legendares.But Roaming Legends(Like Mesprit)Are the real ones you want to use it on.
Mesprit or Cressalia. They are both running Pokemon, so they are very hard to catch.