the best way is to bring it down to the red mark a bring about 10-15 revives, 20 max potions, 25 quick balls, 19 dark balls, 15 ultra balls, 15 great balls, but you can use a masterball on it
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you catch raikou on the last gym leader in the kanto leaders and you catch uxie in vermillion city from
yes uxie comes back to life after a while.
When you catch dialga/palkia or defeat them, you can catch azelf and uxie in the lakes.Mespirit will flee so you gotta get a poketech radar to track it down.
wait for a long time then try again (it helps to save before you "talk" to uxie, this way if you defeat it all you have to do is turn off your DS system and try again)
That would be a Master Ball, however, if you don't have it no more, or want to save it, use a duskball or a Ultraball