i dont think you can
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You cannot find a deoxys normally(during game play). but you can get it during an event. first you have to get a mystery gift. if you do not have the mystery gift, it is explained on the Pokemon website. there is no deoxys event currently going on.
Deoxys' Forme randomly changes as you progress through a dungeon. The changes are purely cosmetic, except for the Speed Forme, which doubles his movement speed until you move to the next floor.
The Luminous Orb, which displays the entire dungeon floor, is obtained in Wish Cave, Buried Relic or Sinister Woods. There may be a chance to get it as a reward for a mission or through the Kecleon Shops (in dungeons and in Pokemon Square).
you have to have action replay and then go to game cheats.com and it will tell you
Buy friend areas. The last dungeon is the deoxys one
19 deoxys is on the the 20 (the real deoxys)
Fight him alone with friend bow
Go to Shimmer Desert floor 20 and you can meet Deoxys(You must bring a Secret Slab.)
First, you have to get a secret slab. Once you have that, go to shimmer hill, 13th floor and find Deoxys, defeat him, and recruit him.
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after Befriending Lugia Meteor cave will open up.
you will have to go up stairs and if your deoxys changes into attack form level it up and then it will learn zap cannon
you get it by entering meteor cave, which is found by blastoise the next morning after recruiting lugia
go to shimmer hill 17F[make sure you have mystery part / secret slab] and find him ps:he' very hard to recruit
Get to F17 in Shimmer Hill with Mystery Part or Secret Slab in bag and then walk until you find Deoxys. Good Luck
darkrai ho-oh deoxys mewtwo zapdos latias latios entei raikou lugia jirachi moltres suicune