first catch palkia. go to lake acuity.there is a cavern in the middle. go inside.there will be uxie level 50.
it is good to have a Pokemon of electric type moves to paralyze the uxie since it will be using "yawn" alot. a move that makes Pokemon sleep on the second turn. make sure you have 20 awakenings and Pokemon close to the level 50. Also if u have the TMs No.54 False swipe this is garunteed to bring uxies life down to 1 hp without making it faint. I caught the uxie witha timer ball. i had 16 and used 6. only after uxie was paralyzed was it eazier to catch it.
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After you catch/defeat palkia/dialga, uxie will be at lake Acuity, in the cave in the middle of the lake.
You can't, you will need to trade them from Pokemon Diamond, Pearl or Platinum.
you can go back and catch: -azelf -Uxie -Mesprit and also a SECOND -Palkia and a SECOND -Dialga
Mespirit, Azelf, and Uxie are in the three lakesin sinnoh. Palkia (and possibly dialga) are in the end of mt. coronet.
memmemem whatever that means,#146 is Uxie
some say you can catch dialga in the celestic cave, that's how i got him.